METHOD OF OPERATING A PARTICLE BEAM SYSTEM AND COMPUTER PROGRAM PRODUCT

Particle beam systems, for example electron beam microscopes, exhibit improved resolution in a first direction by manipulating a beam of charged particles so that the beam has a non-circular beam profile in a focal plane of an objective lens. Multiple images of a sample can be recorded at different orientations of the beam profile relative to the sample, and the recorded images can be synthesized using non-uniform spatial-frequency weights to obtain an image of the sample having improved resolution in any direction. The orientation of the beam profile can be adjusted to a target orientation depending on a structure on a sample prior to recording an image of the sample, thereby making it possible to achieve highest resolution in a selected direction of interest.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit under 35 U.S.C. § 119 to German Application No. 10 2023 102 073.0, filed Jan. 27, 2023. The entire disclosure of this application is incorporated by reference herein.

FIELD

The present disclosure relates to particle beam systems and methods for operating the same. Further, the present disclosure relates to a computer program product for executing the method on a particle beam system.

BACKGROUND

Some conventional particle beam systems comprise a particle source for generating a beam of charged particles, an aperture stop having a circular aperture limiting the beam laterally, an objective lens focusing the beam into a focal plane, a detector detecting interaction products of the beam with a sample located in the focal plane and outputting a detection signal according to the detected interaction products, and a controller controlling the particle beam system and evaluating the detection signal to generate an image of the sample.

However, the focusing of the beam by the objective lens can cause spherical aberration and chromatic aberration. For conventional certain electron beam microscopes, the energy width of the beam can have values in the order of 0.3 eV to 0.7 eV for specific particle sources of high performance, while particle sources with an energy width of the beam of up to 3 eV are in use. Accordingly, chromatic aberration can be the dominant contribution to a total aberration of the beam in the focal plane. In this case, chromatic aberration can be the dominant contribution to limited resolution. In general, the smaller the total aberration of the beam in the focal plane, the better the resolution.

For reducing the chromatic aberration of the focusing of the beam by the objective lens, a conventional corrector can be used. A conventional corrector is illustrated inFIG.1. This conventional corrector has four sequentially arranged multipole field generators each configured to generate a multipole field for manipulating the beam of charged particles. A collimated beam having a circular cross-section enters the first multipole field parallel to a z-direction which coincides with an optical axis of the conventional corrector. The first multipole field focuses the beam in an x-direction and defocuses the beam in a y-direction.

The second multipole field is arranged in z-direction at a focal plane for the x-direction of the first multipole field. The second multipole field has an electric quadrupole field and a magnetic quadrupole field superimposed on each other. The second multipole field only weakly affects the beam in the x-direction because the beam crosses the optical axis at the second multipole field. The second multipole field focuses the beam in the y-direction thereby correcting a chromatic aberration in the y-direction of a subsequent objective lens.

The third multipole field is arranged in z-direction at a focal plane for the y-direction of the second multipole field. The third multipole field has an electric quadrupole field and a magnetic quadrupole field superimposed on each other. The third multipole field only weakly affects the beam in the y-direction because the beam crosses the optical axis at the third multipole field. The third multipole field focuses the beam in the x-direction thereby correcting a chromatic aberration in the x-direction of the subsequent objective lens.

The fourth multipole field defocuses the beam in the x-direction and focuses the beam in the y-direction thereby generating a collimated beam having a circular cross-section.

SUMMARY

The above-described conventional corrector has a relatively complex structure and can be difficult to adjust and to operate. Further, the conventional corrector can be relatively expensive, e.g., due to the large number of electric and magnetic fields to be generated and to be adjusted. The conventional corrector is also relatively large.

The present disclosure seeks to provide a relatively simpler and relatively inexpensive particle beam system capable of recording an image of a sample.

A first aspect of the disclosure relates to a method of operating a particle beam system, the method comprising: generating a beam of charged particles; focusing the beam into a focal plane by an objective lens; manipulating the beam into a beam profile in which a ratio of a first interaction length to a second interaction length amounts to at most 1:1.2, wherein the first interaction length is a distance, measured along a first direction, between a first straight line perpendicular to the first direction and a second straight line perpendicular to the first direction, wherein the first straight line defines a first half-plane, wherein the first half-plane is located in the focal plane and contains 25% of a total intensity of the beam in the focal plane, wherein the second straight line defines a second half-plane, wherein the second half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane and does not overlap the first half-plane, wherein the second interaction length is a distance, measured along a second direction different from the first direction, between a third straight line perpendicular to the second direction and a fourth straight line perpendicular to the second direction, wherein the third straight line defines a third half-plane, wherein the third half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane, wherein the fourth straight line defines a fourth half-plane, wherein the fourth half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane and does not overlap the third half-plane; adjusting an orientation of the beam profile in the focal plane relative to a sample to a target orientation; recording an image of the sample located in the focal plane using the manipulated beam having the adjusted orientation; repeating the adjusting and the recording with at least one other target orientation different from the previously used target orientations; and calculating a synthesized image of the sample based on the recorded images.

According to the first aspect, the first interaction length, which characterizes the intensity distribution of the beam in the first direction in the focal plane, is smaller than the second interaction length, which characterizes the intensity distribution of the beam in the second direction in the focal plane. For example, the first interaction length is smaller than a corresponding interaction length of a beam generated by a conventional particle beam system not having any specific correctors. Consequently, a maximum resolution in the first direction in the image(s) recorded by the method according to the first aspect can be higher than a maximum resolution of an image recorded by a conventional particle beam system not having any specific correctors. This feature generally comes at the cost that a maximum resolution in the second direction of the image(s) recorded by the method according to the first aspect is worse than the maximum resolution of the image recorded by the conventional particle beam system not having any specific correctors. Consequently, in comparison to a conventional particle beam system not having any specific correctors, the synthesized image obtained by the method according to the first aspect can provide improved resolution in any direction.

In comparison to the conventional particle beam system having the above-described conventional corrector, a system for performing the method according to the first aspect comprises less components while providing similar functionality.

A second aspect of the disclosure relates to a method of operating a particle beam system, the method comprising: generating a beam of charged particles; focusing the beam into a focal plane by an objective lens; manipulating the beam into a beam profile in which a ratio of a first interaction length to a second interaction length amounts to at most 1:1.2, wherein the first interaction length is a distance, measured along a first direction, between a first straight line perpendicular to the first direction and a second straight line perpendicular to the first direction, wherein the first straight line defines a first half-plane, wherein the first half-plane is located in the focal plane and contains 25% of a total intensity of the beam in the focal plane, wherein the second straight line defines a second half-plane, wherein the second half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane and does not overlap the first half-plane, wherein the second interaction length is a distance, measured along a second direction different from the first direction, between a third straight line perpendicular to the second direction and a fourth straight line perpendicular to the second direction, wherein the third straight line defines a third half-plane, wherein the third half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane, wherein the fourth straight line defines a fourth half-plane, wherein the fourth half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane and does not overlap the third half-plane; adjusting an orientation of the beam profile in the focal plane relative to a sample to a target orientation; recording an image of the sample located in the focal plane using the manipulated beam having the adjusted orientation; and selecting the target orientation based on an orientation of a structure on the sample.

According to the second aspect, similar to the first aspect, a maximum resolution in the first direction in the image recorded by the method according to the second aspect is higher than a maximum resolution of an image recorded by a conventional particle beam system not having any specific correctors. Consequently, in comparison to a conventional particle beam system not having any specific correctors, the recorded image obtained by the method according to the second aspect has an improved resolution in the target orientation which is a selected direction of particular interest.

A third aspect of the disclosure relates to a computer program product comprising instructions which, when executed by a controller of a particle beam system, causes the particle beam system to perform the method according to the first and second aspect, respectively.

DETAILED DESCRIPTION

Hereinafter, specific embodiments of the disclosure are described in detail with reference to the accompanying drawings. Same or similar elements in different drawings are denoted by same reference numerals.

First Embodiment: Chromatic Aberration Correction in One Direction

FIG.2shows a schematic illustration of a particle beam system100such as an electron beam microscope according to a first embodiment. The particle beam system100can be used to record an image of a sample2.

The particle beam system100comprises a beam column10configured to generate a beam3of charged particles and to direct the generated beam3to the sample2.

The beam column10comprises a particle source11configured to provide the charged particles of the beam3. In case of an electron beam column, the particle source11is an electron source. Typically, an energy width (i.e., full width at half maximum of a histogram of kinetic energies) of an electron beam can have values in the order of 0.3 eV to 0.7 eV for specific electron sources of high performance, while electron sources providing an energy width of the electron beam of up to 3 eV are in use. In case of an ion beam column, the particle source11is an ion source. Typically, an energy width of an ion beam can have values in the order of up to 10 eV depending on the particular type of source. For example, a liquid metal ion source can provide an energy width of approximately 5 eV, a gas field ion source can provide an energy width of approximately 0.3 eV, and a plasma ion source can provide an energy width of approximately 10 eV.

An electric potential applied to the particle source11can be different from an electric potential of the sample2, so that the charged particles of the beam3hit the sample2with a landing energy according to the difference between the electric potential of the particle source11and the electric potential of the sample2.

In practice, for a scanning electron microscope, landing energies of the charged particles of the beam3(i.e., the kinetic energies of the charged particles of the beam3at impact on the sample2) range from a low-value regime in the order of 1 keV to a high-value regime in the order of 10 keV to 20 keV.

An electric current of the charged particles of the beam3is selected as is typical for charged particle beam microscopes. That is, the electric current of the charged particles of the beam3is in the order of 1 pA to 100 pA. Accordingly, the electric current of the charged particles of the beam3is much less than a typical electric current of a charged particle beam of an inspection system in the art. Nevertheless, even the performance of an inspection system with a high electric current can be improved with the procedures of this disclosure.

Alternatively, the electric current of the charged particles of the beam3is selected as is typical for a charged particle beam of an inspection system. That is, the electric current of the charged particles of the beam3is in the order of 500 pA to 100 nA. In this case, the electric current of the charged particles of the beam3is much greater than a typical electric current of a charged particle beam of a charged particle beam microscope in the art.

The beam column10further comprises an acceleration electrode12configured to accelerate the charged particles of the beam3to a selectable kinetic energy in accordance with a selectable electric potential applied to the acceleration electrode12and the electric potential of the particle source11. In the illustrated example, the acceleration electrode12is embodied by a plate electrode having an aperture through which the beam3propagates. In practice, for a scanning electron microscope, the charged particles of the beam3can be accelerated to values within 5 keV to 30 keV. For a typical transmission electron microscope, the acceleration electrode12is grounded and the charged particles of the beam3are accelerated to values above 50 keV.

The beam column10further comprises an objective lens13configured to focus the beam3into a focal plane FP. The objective lens13can be a magnetic and/or electrostatic lens, for example. The focal plane FP is indicated by a dashed line inFIG.2. For example, the objective lens13spherically focuses the beam3into the focal plane FP. This means that the objective lens13focuses a collimated beam having a circular beam shape propagating parallel to an optical axis OA of the objective lens13into a circular spot on the focal plane FP. In this context, a beam has a circular beam shape if a beam contour corresponding to a limiting aperture stop has a circular shape. In other words, the objective lens13is configured to generate a focusing field acting on the beam3, wherein the focusing field is substantially radially symmetric about the optical axis OA.

The beam column10further comprises a beam deflection system14configured to deflect the beam3so that the beam3can be directed to a plurality of locations on the sample2. The beam deflection system14can be configured to deflect the beam3along two directions which are substantially perpendicular to each other and to an optical axis OA of the objective lens13. In the example illustrated inFIG.2, the optical axis OA of the objective lens13is parallel to the vertical axis ofFIG.2and is located at a center of the objective lens13. The optical axis OA of the objective lens13is perpendicular to the focal plane FP.

The beam column10further comprises a first multipole field generator18. The first multipole field generator18is configured to generate a first multipole field for manipulating the beam3. The first multipole field acts on the beam3downstream (i.e., in direction from the particle source11to the focal plane FP behind) the acceleration electrode12. The first multipole field acts on the beam3upstream (i.e., in direction from the particle source11to the focal plane FP before) the objective lens13. The function of the first multipole field is described further below with reference toFIGS.3and4.

The beam column10can further comprise one or more condenser lenses (not illustrated in the figures) that are arranged between the particle source11and the objective lens3, or between the acceleration electrode12and the first multipole field generator18. Typically, condenser lenses can be used to change the probe current or to adapt the illumination angle of the charged particle beam3.

The particle beam system100further comprises a vacuum chamber20. The vacuum chamber20has a chamber wall21defining the vacuum chamber20. The vacuum chamber20is connected to the beam column10. A vacuum can be generated inside the vacuum chamber20. The beam3can enter the vacuum chamber20through an opening22encompassed by the objective lens13.

The particle beam system100further comprises a sample stage4configured to hold the sample2. The sample stage4can be configured to displace the sample2in one or more directions. The sample stage4can be configured to rotate the sample2about one or more axes of rotation. For example, the sample stage4can be configured to rotate the sample2about two or three axes of rotation. For example, the axes of rotation can be orthogonal to each other. The sample stage4is located inside the vacuum chamber20.

The particle beam system100further comprises a controller30configured to control all or some of components of the particle beam system100. For example, the controller30can be configured to control the beam column10via a signal connection15and the sample stage4via a signal connection5. Specifically, the controller30can control the particle source11, the acceleration electrode12(e.g., by controlling an electric potential applied to the acceleration electrode12), the objective lens13(e.g., by controlling electric potential(s) and/or electric current(s) applied to the objective lens13) and the beam deflection system14(e.g., by controlling electric potential(s) and/or electric current(s) applied to the beam deflection system14).

The particle beam system100further comprises a data memory31configured to store data. The controller30can read data from the data memory31and write data into the data memory31.

The particle beam system100further comprises an output device32configured to output information. For example, the output device32can be a display device for displaying information provided by the controller30.

The particle beam system100further comprises an input device33for providing instructions to the controller30. The input device33can comprise a mouse, a keyboard and the like, for example. The input device33can comprise a data interface for communication with another communication device.

The particle beam system100further comprises a detection system40configured to detect interaction products41emerging from the sample2due to an interaction of the beam3with the sample2. For example, the detection system40can comprise at least one of a detector for detecting charged particles, such as backscattered ions, backscattered electrons, or secondary electrons, and a detector for detecting radiation such as light and x-rays. The detection system40is further configured to generate a detection signal based on the detected interaction products, to output the detection signal to the controller30via a signal connection42, and to be adjusted by the controller30via the signal connection42.

With reference toFIGS.3and4, the function of the first multipole field generated by the first multipole field generator18is described below.FIG.3shows a schematic illustration of a trajectory of a fundamental ray6in a first plane defined by a first direction and a z-axis.FIG.4shows a schematic illustration of the trajectory of the fundamental ray6in a second plane defined by a second direction and the z-axis. The z-axis is chosen to correspond to the optical axis OA of the objective lens13. The first direction and the second direction are specific directions defined in dependence of the beam profile of the beam3in the focal plane FP. The first direction is perpendicular to the z-axis. The second direction is perpendicular to the z-axis. The first direction and the second direction rotate about the z-axis in accordance with any Larmor precession present. That is, when observed from a stationary coordinate system, the first direction and the second direction rotate about the z-axis in dependence of the z-coordinate. Consequently, when observed from a stationary coordinate system, the first plane would be considered not a flat plane but helical; and the second plane would be considered not a flat plane but helical. In contrast, in a rotating coordinate system rotating about the z-axis with the Larmor precession, the first plane would be considered a flat plane; and the second plane would be considered a flat plane.FIGS.3and4show the first plane and the second plane in this rotating coordinate system, respectively. A particular definition of the first direction and the second direction is described with reference toFIGS.5A and5B.

In the example ofFIGS.3and4, the first direction is assumed to correspond to an x-axis; and the second direction is assumed to correspond to a y-axis. However, this selection is chosen for the purpose of explanation. The x-axis is perpendicular to the z-axis. The y-axis is perpendicular to both the x-axis and the z-axis. The first plane corresponds to the xz-plane; and the second plane corresponds to the yz-plane. The first plane (xz-plane) is perpendicular to the focal plane FP. The second plane (yz-plane) is perpendicular to the focal plane FP. Planes are perpendicular to each other if their respective normals (normal vectors) are perpendicular to each other.

The fundamental ray6passes through a circular aperture16of an aperture stop17. The aperture stop17blocks a portion of the beam3and transmits another portion of the beam3through the aperture16. The aperture stop17can be an additional component of the beam column10or can be functionally embodied by one of the other components of the beam column10, such as the acceleration electrode12, for example.

The objective lens13focuses the fundamental ray6having a predetermined kinetic energy En into the focal plane FP where the sample2is located. The trajectory of the fundamental ray6having the predetermined kinetic energy En is illustrated as a solid line. However, generally, the objective lens13causes chromatic aberration, i.e., the focusing differs for charged particles having different kinetic energies.

In order to compensate the chromatic aberration of the objective lens13in the first direction (i.e., in the first plane), the first multipole field generated by the first multipole field generator18provides an energy-dispersive focusing/defocusing. That is, the first multipole field generator18is configured and operated so that the chromatic aberration of the focusing by the objective lens13in the first direction (x-axis) is reduced and that the chromatic aberration of the focusing by the objective lens13in the second direction (y-axis) is increased.

For example, as illustrated inFIG.3, charged particles having the predetermined kinetic energy En (illustrated by a solid line), charged particles having a kinetic energy El (illustrated by a dashed line) less than the predetermined kinetic energy En and charged particles having a kinetic energy Eh (illustrated by a dotted line) greater than the predetermined kinetic energy En are focused by the first multipole field in the first direction (x-axis) with different powers so that an energy-dispersive focusing of the objective lens13(i.e., chromatic aberration) is compensated. Further, as illustrated inFIG.4, charged particles having the predetermined kinetic energy En (illustrated by a solid line), charged particles having the kinetic energy El (illustrated by a dashed line) less than the predetermined kinetic energy En and charged particles having the kinetic energy Eh (illustrated by a dotted line) greater than the predetermined kinetic energy En are focused by the first multipole field in the second direction (y-axis) with different powers so that the energy-dispersive focusing of the objective lens13(i.e., chromatic aberration) is not compensated but even increased.

Note that the expression “focusing” represents both of converging focusing, i.e., focusing with a positive optical power, and diverging focusing, i.e., focusing with a negative optical power, also referred to as defocusing. Accordingly, converging focusing is focusing with a power greater than that of diverging focusing; and diverging focusing (or defocusing) is focusing with a power less than that of converging focusing.

Referring toFIG.3, the first multipole field generator18is operated to focus the charged particles having the kinetic energy El (illustrated by a dashed line) in the first plane (xz-plane) with a power which is smaller than a power with which the first multipole field generator18focuses the charged particles having the kinetic energy En (illustrated by a solid line) in the first plane (xz-plane). In the example ofFIG.3, the first multipole field generator18is operated to defocus (i.e., focus with negative power) the charged particles having the kinetic energy El (illustrated by a dashed line) in the first plane (xz-plane), whereas the charged particles having the kinetic energy En (illustrated by a solid line) are not focused (i.e., focused with power of 0) in the first plane (xz-plane). Further, the first multipole field generator18is operated to focus the charged particles having the kinetic energy Eh (illustrated by a dotted line) in the first plane (xz-plane) with a power which is greater than the power with which the first multipole field generator18focuses the charged particles having the kinetic energy En (illustrated by a solid line) in the first plane (xz-plane). In the example ofFIG.3, the first multipole field generator18is operated to focus (i.e., focus with positive power) the charged particles having the kinetic energy Eh (illustrated by a dotted line) in the first plane (xz-plane), whereas the charged particles having the kinetic energy En (illustrated by a solid line) are not focused (i.e., focused with power of 0) in the first plane (xz-plane).

Referring toFIG.4, the first multipole field generator18is operated to focus the charged particles having the kinetic energy Eh (illustrated by a dotted line) in the second plane (yz-plane) with a power which is smaller than a power with which the first multipole field generator18focuses the charged particles having the kinetic energy En (illustrated by a solid line) in the second plane (yz-plane). In the example ofFIG.4, the first multipole field generator18is operated to defocus (i.e., focus with negative power) the charged particles having the kinetic energy Eh (illustrated by a dotted line) in the second plane (yz-plane), whereas the charged particles having the kinetic energy En (illustrated by a solid line) are not focused (i.e., focused with power of 0) in the second plane (yz-plane). Further, the first multipole field generator18is operated to focus the charged particles having the kinetic energy El (illustrated by a dashed line) in the second plane (yz-plane) with a power which is greater than the power with which the first multipole field generator18focuses the charged particles having the kinetic energy En (illustrated by a solid line) in the second plane (yz-plane). In the example ofFIG.4, the first multipole field generator18is operated to focus (i.e., focus with positive power) the charged particles having the kinetic energy El (illustrated by a dashed line) in the second plane (yz-plane), whereas the charged particles having the kinetic energy En (illustrated by a solid line) are not focused (i.e., focused with power of 0) in the second plane (yz-plane).

Note that the effect on the trajectories of the charged particles achieved by the first multipole field generator18is illustrated in an exaggerated fashion for the purpose of illustration. That is, the amount of change in propagation direction of charged particles effected by the first multipole field generator18is exaggerated in comparison to the real effect.

The focusing by the objective lens13is achieved by a magnetic and/or electric field. A focusing power of the focusing by the objective lens13increases with decreasing kinetic energy of the charged particles. Consequently, in the first plane (xz-plane) illustrated inFIG.3, due to their decreased kinetic energy, the charged particles of the fundamental ray6having the kinetic energy El exhibit a stronger focusing by the objective lens13than the charged particles of the fundamental ray6having the predetermined kinetic energy En; and the charged particles of the fundamental ray6having the predetermined kinetic energy En exhibit a stronger focusing by the objective lens13than the charged particles of the fundamental ray6having the kinetic energy Eh. Consequently, when appropriately excited, the first multipole field reduces the chromatic aberration of the focusing by the objective lens13in the first direction (x-axis).FIG.3illustrates an example in which the first multipole field is excited so as to completely compensate the chromatic aberration in the first direction.

However, as illustrated inFIG.4, the energy-dispersive focusing/defocusing of the first multipole field introduces additional chromatic aberration on top of the chromatic aberration of the focusing by the objective lens13in the second direction (y-axis). For example, the first multipole field focusses the charged particles of the fundamental ray6having the kinetic energy El (illustrated as a dashed line) greater than the charged particles of the fundamental ray6having the predetermined kinetic energy En in the second direction (y-axis). Further, the first multipole field focusses the charged particles of the fundamental ray6having the kinetic energy Eh (illustrated as a dotted line) less (i.e., the first multipole field defocusses the charged particles of the fundamental ray6having the kinetic energy Eh greater) than the charged particles of the fundamental ray6having the predetermined kinetic energy En in the second direction (y-axis).

Effect in Focal Plane FP, Beam Profile

Referring toFIGS.5and6, the effect of the first multipole field on the beam3is further described.FIGS.5A and5Bshow a schematic illustration of a beam profile34of the beam3in the focal plane FP. The beam profile34of the beam3in the focal plane FP is schematically illustrated by three isolines and a point O, which are described below. The point O represents a center of an intensity distribution of the beam3in the focal plane FP. For example, the center of the intensity distribution of the beam3in the focal plane FP can be determined using the formula

wherein {right arrow over (O)} represents the center of the intensity distribution of the beam3in the focal plane FP, F represents a spatial vector in the focal plane FP, l({right arrow over (r)}) represents the intensity distribution in dependence of the spatial vector {right arrow over (r)}, Itotalrepresents a total intensity of the beam3in the focal plane FP. The Integration is performed over the entire focal plane FP. For example, Itotalcan be determined using the formula

Each isoline represents locations in the focal plane FP at which the local intensity of the beam3amounts to a same value. In the illustrated example, the isolines have an elliptic shape and are centered on a center O of the intensity distribution of the beam3in the focal plane FP. For example, the isoline closest to the center O is illustrated by a solid line and represents locations in the focal plane FP at which the local intensity of the beam3amounts to ¾ Imax, wherein Imaxrepresents the maximum value of the intensity distribution of the beam3in the focal plane FP. Further, the isoline second-closest to the center O is illustrated by a dashed line and represents locations in the focal plane FP at which the local intensity of the beam3amounts to ½ Imax. Finally, the isoline furthest from the center O is illustrated by a dotted line and represents locations in the focal plane FP at which the local intensity of the beam3amounts to ¼ Imax. At the center O, the local intensity of the beam3amounts to Imax.

Hereinafter, the beam profile34of the beam3in the focal plane FP will be characterized by a direction-specific interaction length defined as follows: An interaction length specific to a direction in the focal plane indicates a distance, measured along the specific direction, between a first straight line perpendicular to the specific direction and a second straight line perpendicular to the specific direction, wherein the first straight line defines a first half-plane, wherein the first half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane (i.e., 25% of Itotal), wherein the second straight line defines a second half-plane, wherein the second half-plane is located in the focal plane and contains 25% of the total intensity of the beam in the focal plane (i.e., 25% of Itotal) and does not overlap the first half-plane.

Accordingly, a strip in the focal plane, including an area between the first straight line and the second straight line, contains 50% of the total intensity Itotalof the beam3in the focal plane FP. The first half-plane and the second half-plane together contain the other 50% of the total intensity Itotalof the beam3in the focal plane FP.

The above definition of the interaction length is applied to the beam profile34of the beam3illustrated inFIG.5Afor a first direction D1in the focal plane FP, resulting in a first interaction length W1for the first direction D1: A first half-plane HP1is located in the focal plane FP and is defined by (i.e., is limited by) a first straight line L1in the focal plane FP. The first straight line L1is perpendicular to the first direction D1. The first half-plane HP1contains 25% of the total intensity of the beam in the focal plane FP. A second half-plane HP2is also located in the focal plane FP and is defined by (i.e., is limited by) a second straight line L2in the focal plane FP. The second straight line L2is also perpendicular to the first direction D1. The second half-plane HP2also contains 25% of the total intensity of the beam in the focal plane FP. The first half-plane HP1and the second half-plane HP2do not overlap each other. The first interaction length W1is the distance, measured along the first direction D1, between the first line L1and the second line L2.

InFIGS.5A and5B, the first half-plane HP1is illustrated by a hatched area. Please note that the first half-plane HP1extends beyond the illustrated hatched area along the positive first direction D1and directions perpendicular to the first direction D1. InFIGS.5A and5B, the second half-plane HP2is also illustrated by a hatched area. Please note that the second half-plane HP2extends beyond the illustrated hatched area along the negative first direction D1and directions perpendicular to the first direction D1.

The above definition of the interaction length is applied to the beam profile34of the beam3illustrated inFIG.5Bfor a second direction D2in the focal plane FP, which is different from the first direction D1, resulting in a second interaction length W2for the second direction D2: A third half-plane HP3is located in the focal plane FP and is defined by (i.e., is limited by) a third straight line L3in the focal plane FP. The third straight line L3is perpendicular to the second direction D2. The third half-plane HP3contains 25% of the total intensity of the beam in the focal plane FP. A fourth half-plane HP4is also located in the focal plane FP and is defined by (i.e., is limited by) a fourth straight line L4in the focal plane FP. The fourth straight line L4is also perpendicular to the second direction D2. The fourth half-plane HP4also contains 25% of the total intensity of the beam in the focal plane FP. The third half-plane HP3and the fourth half-plane HP4do not overlap each other. The second interaction length W2is the distance, measured along the second direction D2, between the third line L3and the fourth line L4.

FIG.6shows a schematic illustration of an interaction profile35of the beam3having the beam profile34illustrated inFIGS.5A and5B. The interaction profile35represents the direction-specific interaction lengths for all directions of the focal plane FP. Each of the interaction lengths is indicated by two points which are arranged on a line oriented along the direction associated with the respective interaction length and are separated by a distance corresponding to the respective interaction length. Consequently, the interaction length W1for the first direction D1is illustrated as two points arranged along the direction D1and separated by a distance W1. Similarly, the interaction length W2for the second direction D2is illustrated as two points arranged along the direction D2and separated by a distance W2. For the beam profile34illustrated inFIGS.5A and5B, this results in an interaction profile35having an elliptic shape when the lines representing the interaction lengths are centered on a common center.

As illustrated inFIGS.5and6, the interaction length W1for the first direction D1is shorter than the interaction length W2for the second direction D2. Consequently, when using the beam3as a scanning microscope probe, in a resulting image, the maximum resolution in the first direction D1is higher than the maximum resolution in the second direction D2. This means that the resolution in the first direction D1is better than the resolution in the second direction D2when the beam3having the illustrated intensity distribution is used for recording an image. Specifically, the beam profile34illustrated inFIGS.5A and5B, resulting in the interaction profile35illustrated inFIG.6, is particularly suited for recording images of a sample having straight microstructures (such as straight edges). For example, when the beam3is orientated so that the first direction D1is perpendicular to the straight microstructures (i.e., the first line L1is parallel to the straight microstructures), the obtained resolution along the first direction D1will be exceptionally good. However, simultaneously, the obtained resolution along the second direction D2will be worse.

Furthermore, a conventional non-corrected particle beam system provides a uniform interaction length WN for all directions in the focal plane, i.e., the beam of the conventional non-corrected particle beam system provides the same interaction length for every direction in the focal plane. Compared to such a conventional non-corrected particle beam system, the interaction length W1for the first direction D1according to the present embodiment is also shorter than the interaction length WN of the conventional non-corrected particle beam system.

This improved resolution along the first direction D1can be beneficial in multiple applications, examples of which are described further below with reference toFIGS.25and26.

A ratio of the first interaction length W1to the second interaction length W2amounts to at most 1:1.2. Optionally, the ratio of the first interaction length W1to the second interaction length W2amounts to at most 1:1.3 or at most 1:2.0 or at most 1:3.0. In the example illustrated inFIG.6, the ratio of the first interaction length W1to the second interaction length W2amounts to approximately 1:3. Within specific limits, when using the beam3having the beam profile illustrated inFIGS.5A and5Bas a scanning microscope probe, the resolution in the first direction D1improves relative to the resolution in the second direction D2with decreasing values of the ratio.

The primary objective of the present disclosure is to reduce the absolute value of first interaction length W1, thereby improving the resolution in the direction D1. The secondary objective of the present disclosure is to achieve the primary objective while maximizing the value of the ratio W1to W2, thereby avoiding an unnecessarily long second interaction length W2.

Implementation of the First Multipole Field Generator18

Exemplary implementations of the first multipole field generator18are described with reference toFIGS.7and8.FIGS.7and8show a cross-section of the first multipole field generator18in a plane coplanar to the focal plane FP, i.e., in a plane perpendicular to the optical axis OA of the objective lens13.

According toFIG.7, the first multipole field generator18comprises four electrodes23for providing electric poles and four coils24for providing magnetic poles. The electrodes23and the coils24are arranged about a common center located at the optical axis OA of the objective lens13. In circumferential direction about the common center, the electrodes23and the coils24are disposed alternately. By appropriate application of voltages and currents to the electrodes23and the coils24, respectively, various types of electric and magnetic multipole fields can be generated. For example, the configuration allows to generate electric and magnetic dipole fields and quadrupole fields of selectable strength. For further reference, the electrodes23generate a first electric multipole field; and the coils24generate a first magnetic multipole field. The first electric multipole field and the first magnetic multipole field are superimposed on each other.

According to a specific mode of operation illustrated inFIG.7, poles facing each other across the optical axis OA of the objective lens13are of the same type, i.e., two positive electric poles denoted “+” face each other across the optical axis OA of the objective lens13, two negative electric poles denoted “−” face each other across the optical axis OA of the objective lens13, two magnetic north poles denoted “N” face each other across the optical axis OA of the objective lens13, and two magnetic south poles denoted “S” face each other across the optical axis OA of the objective lens13. Accordingly, the first electric multipole field is a quadrupole field; and the first magnetic multipole field is a quadrupole field. In general, the first electric multipole field has a four-pole component, and the first magnetic multipole field has a four-pole component.

Solid lines inFIG.7schematically indicate different field lines of the first electric multipole field. Solid arrows inFIG.7indicate forces generated by the first electric multipole field on a charged particle, in the illustrated example an electron, (of the beam3) at the respective locations. The magnitude of the force generated by the first electric multipole field increases (linearly) with increasing distance from the optical axis OA. Dashed lines inFIG.7schematically indicate different flux lines of the first magnetic multipole field. Hollow arrows inFIG.7indicate forces generated by the first magnetic multipole field on a charged particle, in the illustrated example an electron, (of the beam3) at the respective locations. The magnitude of the force generated by the first magnetic multipole field increases (linearly) with increasing distance from the optical axis OA.

While the force generated by the first electric multipole field does not depend on the velocity (kinetic energy) of the charged particles, the force generated by the first magnetic multipole field does depend on the kinetic energy (does depend linearly on the velocity) of the charged particles. Consequently, the net force acting on a charged particle depends on the kinetic energy (does depend linearly on the velocity) of the charged particle. Further, the net force acting on a charged particle depends (linearly) on the distance of the charged particle from the optical axis OA.

For example, the first electric multipole field and the first magnetic multipole field can be generated so that charged particles of the beam3having the predetermined energy En are not influenced, i.e., the force generated by the first electric multipole field integrated along propagation paths of the charged particles of the beam3having the predetermined energy En and the force generated by the first magnetic multipole field integrated along the propagation paths practically cancel each other for charged particles having the predetermined energy En. In contrast to that, charged particles having a kinetic energy Eh greater than the predetermined energy En exhibit a net force along the direction of the hollow arrow representing the force generated by the first magnetic multipole field; and charged particles having a kinetic energy El less than the predetermined energy En exhibit a net force along the direction of the solid arrow representing the force generated by the first electric multipole field. Consequently, charged particles having a kinetic energy Eh greater than the predetermined energy En are focused in the first direction (x-axis) and are defocused in the second direction (y-axis); and charged particles having a kinetic energy El less than the predetermined energy En are defocused in the first direction (x-axis) and are focused in the second direction (y-axis).

While the first multipole field generator18described with reference toFIG.7comprises four electrodes23and four coils24, other implementations of the first multipole field generator18can comprise more than four electrodes and more than four coils.

As illustrated inFIG.8, the first multipole field generator18can comprise eight electrodes23for providing electric poles and eight coils24for providing magnetic poles. The electrodes23and the coils24are arranged about a common center located at the optical axis OA of the objective lens13. By appropriate application of voltages and currents to the electrodes23and the coils24, respectively, various types of electric and magnetic multipole fields can be generated. For example, the configuration allows to generate electric and magnetic dipole fields and quadrupole fields of selectable strength and selectable orientation as well as electric and magnetic octupole fields of selectable strength.

The coils24can be wound around a non-magnetic material. Alternatively, the coils24can be wound around magnetic pole pieces25guiding the magnetic field generated by the coils24and increasing the magnetic field strength at the optical axis OA of the objective lens13. The pole pieces25can be electrically isolated by a high-voltage isolation or by (vacuum) gaps. The magnetic pole pieces25can also serve as the electrodes23. Some of the electrodes23can be supplied by the same voltage supply, thereby reducing the amount of independent voltage sources and increasing stability of the generated field at the cost of reduced flexibility. For example, electrodes arranged opposite to each other across the optical axis OA can be supplied by a same voltage source. Some of the coils24can be connected to each other, for example connected in series, thereby reducing the amount of independent current sources and increasing stability of the generated field at the cost of reduced flexibility. For example, coils arranged opposite to each other across the optical axis OA can be supplied by a same current source. An additional yoke26surrounding the coils24on the radial outside can be provided for supporting the magnetic flux (“closing” the magnetic flux path) and further increasing the magnetic field strength at the optical axis OA of the objective lens13.

By appropriate application of voltages and currents to the electrodes23and the coils24of the first multipole field generator18illustrated inFIG.8, the first multipole field generator18can generate the first electric multipole field as a quadrupole field and the first magnetic multipole field as a quadrupole field and can selectively rotate both about the optical axis OA, thereby adjusting the orientation of the first multipole field (i.e., rotating the first multipole field). Accordingly, an orientation of the beam profile in the focal plane FP relative to the sample3can be adjusted to a target orientation by rotating the first multipole field.

WhileFIG.7describes the first multipole field generator18as to comprise four electrodes and four coils andFIG.8describes the first multipole field generator18as to comprise eight electrodes and eight coils, other implementations of the first multipole field generator18can comprise even more electrodes and coils. For example, the first multipole field generator18can comprise twelve or sixteen electrodes and coils. Consequently, rotatable multipole fields of higher order, such as a rotatable electric octupole field and a rotatable magnetic octupole field, can be generated for higher order manipulations of the beam3.

In order to reduce a contribution to focusing, to spherical aberration Cs, and to four-fold astigmatism C4at a fringing field region (i.e., where charged particles enter or leave the multipole field), and in order to increase the multipole field strength at a certain excitation, the first multipole field generator18can have a lengthy configuration. For example, an aspect ratio of the first multipole field generator18, defined as a ratio of (1.) the length of the first multipole field generator18along the optical axis OA to (2.) the bore diameter of the first multipole field generator18perpendicular to the optical axis OA can amount to values ranging from 5 to 100, for example.

As can be understood fromFIGS.2to4, except for the first multipole field generated by the first multipole field generator18, no other electric or magnetic fields are provided for correcting the chromatic aberration of the focusing by the objective lens13in a direction different from the first direction (x-axis). This simplifies the structure of the particle beam system100and reduces its costs.

Second Embodiment: Chromatic Aberration Correction in One Direction & Non-Circular Aperture Stop

As described above with reference toFIGS.3and4, the aperture stop17has a circular aperture16. Consequently, the beam3has a circular beam shape in a plane OL which is coplanar to the focal plane FP and located at the position of the objective lens13. Further, the first multipole field generator18does not focus/defocus the charged particles having the predetermined kinetic energy En of the beam3or provides a uniform focusing/defocusing along the first direction (x-axis) and the second direction (y-axis). Consequently, as illustrated inFIGS.3and4, a maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and a maximum illumination angle ϑyof the beam3in the second plane (yz-plane) for charged particles having the predetermined kinetic energy En are the same.

The maximum illumination angle of the beam3in a given plane is defined as a largest one of all landing angles of the charged particles having the predetermined kinetic energy En of the beam3in the given plane, wherein the landing angle of a charged particle is defined as an angle between the trajectory of the charged particle at the focal plane FP and a normal vector of the focal plane FP.

However, the inventors understood that adjusting the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) to a first maximum illumination angle value and adjusting the maximum illumination angle ϑyof the beam3in the second plane (yz-plane) to a second maximum illumination angle value different from the first maximum illumination angle value can further reduce the first interaction length W1for the first direction D1. This effect is utilized in the second embodiment described below. Further explanations of the effect are provided below with reference to the particle beam system500and the optimization routines and procedures.

In order to utilize this understanding, in addition to the features of the particle beam system100according to the first embodiment, a particle beam system200according to the second embodiment further comprises a mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane).

The second embodiment will be described with reference toFIGS.9and10. Similar toFIGS.3and4,FIGS.9and10show the first and second plane in a rotating coordinate system rotating with the Larmor precession, respectively. That is, although illustrated in a flat plane, the x-axis and the y-axis rotate about the z-axis in dependence of the z-coordinate in accordance with the Larmor precession.

The particle beam system200according to the second embodiment is obtained by replacing the aperture stop17having the circular aperture16of the particle beam system100according to the first embodiment by an aperture stop19having a non-circular aperture such as an elliptical aperture. In the example illustrated inFIGS.9and10, the aperture of the aperture stop19is wider along the first direction (x-axis,FIG.9) than along the second direction (y-axis,FIG.10). Consequently, a maximum angle between the optical axis OA of the objective lens13and the trajectory of the fundamental ray6of the beam3behind the aperture stop19in the first plane (xz-plane) is (much) greater than a maximum angle between the optical axis OA of the objective lens13and the trajectory of the fundamental ray6of the beam3behind the aperture stop19in the second plane (yz-plane). The aperture stop19having the non-circular aperture generates a non-circular beam shape of the beam3in the plane OL and causes the first and second maximum illumination angle values to be different. In this context, a beam has a non-circular beam shape if a beam contour corresponding to a limiting aperture stop has a non-circular shape. Herein, the expressions beam contour and beam shape are used synonymously. Herein, the above effect is referred to as direction-dependent widening of the beam3which means that widening of the beam3in the first plane (xz-plane) is different from widening of the beam3in the second plane (yz-plane).

Similar to the first embodiment, the first multipole field generator18provides energy-dispersive focusing/defocusing. Similar to the first embodiment, the first multipole field generator18is configured and operated so that the chromatic aberration of the focusing by the objective lens13in the first direction (x-axis) is reduced and that the chromatic aberration of the focusing by the objective lens13in the second direction (y-axis) is increased.

Effect of Beam Widening in Plane OL

FIG.11shows a schematic illustration of a plurality of energy-specific beam contour (i.e., a plurality of energy-specific beam contours) of the beam3in the plane OL in accordance with the trajectories illustrated inFIGS.9and10. In this context, an energy-specific beam contour is a beam contour for a beam has only particles having the specific (kinetic) energy.

For example,FIG.11shows an energy-specific beam contour of the charged particles having the predetermined kinetic energy En (illustrated by a solid line), an energy-specific beam contour of the charged particles having the kinetic energy El (illustrated by a dashed line) less than the predetermined kinetic energy En, and an energy-specific beam contour of the charged particles having the kinetic energy Eh (illustrated by a dotted line) greater than the predetermined kinetic energy En. Although not drawn to scale,FIG.11yet illustrates that the influence of the energy-dispersive focusing/defocusing of the first multipole field generator18on the energy-specific beam contours is much smaller than the influence of the direction-dependent widening by the aperture stop19having the non-circular aperture. In other words, the effect of the direction-dependent widening by the aperture stop19having the non-circular aperture is much greater than the effect by the energy-dispersive focusing of the first multipole field generator18. However, in order to illustrate the principle of both effects, the illustrated deflection caused by the energy-dispersive focusing of the first multipole field generator18is exaggerated.

In the illustrated example, the beam3has elliptical energy-specific beam contours with a long axis along the first direction (x-axis) and a short axis along the second direction (y-axis). An aspect ratio of an (energy-specific) beam contour in the plane OL is defined as a ratio of the diameter of the beam contour in the first direction (x-axis) to the diameter of the beam contour in the second direction (y-axis). As illustrated inFIG.9, a large diameter of the beam3in the plane OL along the first direction (x-axis) causes a large maximum illumination angle ϑxin the first plane (xz-plane). As illustrated inFIG.10, a small diameter of the beam3in the plane OL along the second direction (y-axis) causes a small maximum illumination angle ϑyin the second plane (yz-plane).

Effect in Focal Plane FP, Beam Profile

Similar to the above embodiment, the particle beam system200according to the second embodiment generates a beam profile having a non-circular interaction profile35in the focal plane FP as illustrated inFIG.6, thus providing the same benefits. Also, the interaction length W1in the first direction D1obtained in the second embodiment is strongly reduced in comparison to the interaction length WN of a conventional particle beam system.

Adjusting the Orientation of the Beam Profile

The aperture stop19having the non-circular aperture can be rotatable about the optical axis OA of the objective lens13, for example, by a controllable actuator. Accordingly, by rotating the aperture stop19and the first multipole field generated by the first multipole field generator18about the optical axis OA of the objective lens13, the orientation of the beam profile of the beam3in the focal plane FP relative to the sample2can be adjusted. The first multipole field can be rotated by rotating the multipole field generator18and/or by appropriate excitation of the individual electrodes/coils of the multipole field generator18.

Third Embodiment: Chromatic Aberration Correction in One Direction & Beam Widening by Separate Field Generators

A particle beam system300according to a third embodiment is described below with reference toFIGS.12and13. The particle beam system300differs from the particle beam system100according to the first embodiment by further comprising a mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane). The particle beam system300differs from the particle beam system200according to the second embodiment by comprising a different mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane).

In the third embodiment, the mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane) is achieved by a two-stage direction-dependent focusing which is implemented by a second multipole field generator50configured to generate a second multipole field (first stage) and a third multipole field generator55configured to generate a third multipole field (second stage). The second multipole field generator50and the third multipole field generator55are disposed downstream of the aperture stop17having the circular aperture (similar to the first embodiment) and upstream of the objective lens13, for example upstream of the first multipole field generator18. In the illustrated example, the second multipole field generator50is disposed downstream of the aperture stop17and upstream of the third multipole field generator55. However, other arrangements are possible. For example, the arrangement of the first multipole field generator18, the second multipole field generator50and the third multipole field generator55can be different.

Similar toFIGS.3and4,FIGS.12and13show the first and second plane in a rotating coordinate system rotating with the Larmor precession. That is, although illustrated in a flat plane, the x-axis and the y-axis rotate about the z-axis in dependence of the z-coordinate in accordance with the Larmor precession.

FIG.12shows trajectories of charged particles of the beam3in the first plane (xz-plane) including the fundamental ray6.FIG.13shows the trajectories of the same charged particles of the beam3in the second plane (yz-plane) including the fundamental ray6.

The second multipole field generator50is configured to generate the second multipole field so that the charged particles of the beam3are focused according to a selectable first focusing power in the first plane (xz-plane) and the charged particles of the beam3are focused according to a selectable second focusing power in the second plane (yz-plane), wherein the first focusing power is less than the second focusing power. For example, the second focusing power is selected to be equal to the negative first focusing power. Accordingly, upon propagation of the beam3between the second multipole field generator50and the third multipole field generator55, the beam3widens along the first direction (x-axis) (much) more than it widens along the second direction (y-axis). The second multipole field comprises at least one of a second electric multipole field and a second magnetic multipole field. The second multipole field can provide an optical power of at least 2 diopters, such as at least 6 diopters, for example at least 10 diopters. Accordingly, the second multipole field is much stronger than a conventional stigmator field. However, the second multipole field is much weaker than the first electric or magnetic multipole field. According to an example, the second multipole field comprises a quadrupole field. According to an example, the second electric multipole field comprises an electric quadrupole field. According to an example, the second magnetic multipole field comprises a magnetic quadrupole field.

The third multipole field generator55is configured to generate the third multipole field so that the charged particles of the beam3are focused according to a selectable third focusing power in the first plane (xz-plane) and the charged particles of the beam3are focused according to a selectable fourth focusing power in the second plane (yz-plane), wherein the third focusing power is greater than the fourth focusing power. For example, the fourth focusing power is selected to be equal to the negative third focusing power. The third multipole field comprises at least one of a third electric multipole field and a third magnetic multipole field. The third multipole field can provide an optical power of at least 2 diopters, such as at least 6 diopters, for example at least 10 diopters. Accordingly, the third multipole field is much stronger than a conventional stigmator field. However, the third multipole field is much weaker than the first electric or magnetic multipole field. According to an example, the third multipole field comprises a quadrupole field. According to an example, the third electric multipole field comprises an electric quadrupole field. According to an example, the third magnetic multipole field comprises a magnetic quadrupole field.

The first to fourth focusing powers define the focusing for the charged particles having the predetermined kinetic energy En. The focusing powers are selected so that an astigmatic effect is avoided. That is, the focusing powers are selected so that, at the objective lens13, the beam3appears to emerge from a single virtual source. In cooperation, the second multipole field generator50and the third multipole field generator55provide selectable direction-dependent widening of the beam3. Due to the selectable direction-dependent widening of the beam3between the second multipole field generator50and the third multipole field generator55, the maximum illumination angle ϑxand the maximum illumination angle ϑycan be manipulated independently. For example, as illustrated in the example, the maximum illumination angle ϑxcan be made (much) greater than the maximum illumination angle ϑy.

Similar to the first and second embodiment, the first multipole field generator18provides energy-dispersive focusing/defocusing. Similar to the first and second embodiment, the first multipole field generator18is configured and operated so that the chromatic aberration of the focusing by the objective lens13in the first direction (x-axis) is reduced and that the chromatic aberration of the focusing by the objective lens13in the second direction (y-axis) is increased.

Effect of Beam Widening in Plane OL

Similar to the second embodiment, the particle beam system300according to the third embodiment generates a beam3having a non-circular beam shape as illustrated inFIG.11, thus providing the same benefits. In contrast to the second embodiment, the effect of the direction-dependent widening can be adapted easily by changing the strengths of the second and third multipole fields generated by the second multipole field generator50and the third multipole field generator55, respectively.

In order to achieve a sufficient aspect ratio of the beam shape of the beam3in the objective lens13, a distance between the second multipole field generator50and the third multipole field generator55along the optical axis OA of the objective lens13is sufficiently large. In some embodiments, aspect ratios of the beam shape of the beam3in the objective lens13provided by the mechanism for independently manipulating the maximum illumination angles range from about 2 to 5. Therefore, the distance between the second multipole field generator50and the third multipole field generator55is about 5 cm to 20 cm.

Effect in Focal Plane FP, Beam Profile

Similar to the above embodiments, the particle beam system300according to the third embodiment generates a beam profile having a non-circular interaction profile35in the focal plane FP as illustrated inFIG.6, thus providing the same benefits. Also, the interaction length W1in the first direction D1obtained in the third embodiment is strongly reduced in comparison to the interaction length WN of a conventional particle beam system.

Implementation of the Second Multipole Field Generator50and the Third Multipole Field Generator55

FIGS.14and15show schematic illustrations of exemplary implementations of the second multipole field generator50and the third multipole field generator55.FIGS.14and15show a cross-section in a plane coplanar to the focal plane FP, i.e., in a plane perpendicular to the optical axis OA of the objective lens13, at the locations of the second multipole field generator50and the third multipole field generator55, respectively.

The following description refers to the second multipole field generator50; however, the description also applies to the third multipole field generator55. That is, the third multipole field generator55can have the same structural configuration as described for the second multipole field generator50. However, the third multipole field generator55and the second multipole field generator50do not have to be implemented by the same structural configuration. For example, in principle, the third multipole field generator55could be implemented as illustrated inFIG.14whereas the second multipole field generator50could be implemented as illustrated inFIG.15.

According toFIG.14, the second multipole field generator50comprises eight electrodes51for providing electric poles. The electrodes51are arranged about a common center located at the optical axis OA of the objective lens13. By appropriate application of voltages to the electrodes51, various types of electric multipole fields can be generated. For example, the configuration allows to generate electric dipole fields and quadrupole fields of selectable strength and orientation. Consequently, the second multipole field generated by the second multipole field generator50can be rotated about the optical axis OA of the objective lens13together with the first multipole field generated by the first multipole field generator18, thereby adjusting the orientation of the beam profile in the focal plane FP relative to the sample2. For further reference, the electrodes51generate a second electric multipole field.

According toFIG.15, the second multipole field generator50comprises eight coils52for providing magnetic poles. The coils52are arranged about a common center located at the optical axis OA of the objective lens13. By appropriate application of currents to the coils52, various types of magnetic multipole fields can be generated. For example, the configuration allows to generate magnetic dipole fields and quadrupole fields of selectable strength and orientation. Consequently, the second multipole field generated by the second multipole field generator50can be rotated about the optical axis OA of the objective lens13together with the first multipole field generated by the first multipole field generator18, thereby adjusting the orientation of the beam profile in the focal plane FP relative to the sample2. For further reference, the coils52generate a second magnetic multipole field.

While the second multipole field generator50illustrated inFIG.14comprises eight electrodes and the second multipole field generator50illustrated inFIG.15comprises eight coils, other implementations of the second multipole field generator50can comprise even more electrodes and coils. For example, the second multipole field generator50can comprise twelve or sixteen electrodes for generating the second electric multipole field. Further or alternatively, the second multipole field generator50can comprise twelve or sixteen coils for generating the second magnetic multipole field. Consequently, multipole fields of higher order can be generated for higher order manipulations of the beam3.

The coils52can be wound around a non-magnetic material or can be coreless. Alternatively, the coils52can be wound around magnetic pole pieces guiding the magnetic field generated by the coils52and increasing the magnetic field strength at the optical axis OA of the objective lens13. Some of the electrodes51can be supplied by the same voltage supply, thereby reducing the amount of independent voltage sources and increasing stability of the generated field at the cost of reduced flexibility. Some of the coils52can be connected to each other, for example connected in series, thereby reducing the amount of independent current sources and increasing stability of the generated field at the cost of reduced flexibility. An additional yoke53surrounding the coils52on the radial outside can be provided for supporting the magnetic flux (“closing” the magnetic flux path) and further increasing the magnetic field strength at the optical axis OA of the objective lens13.

Fourth Embodiment: Chromatic Aberration Correction in One Direction & Beam Widening by Same Field Generator

A particle beam system400according to a fourth embodiment is described below with reference toFIGS.16and17. The fourth embodiment presents another different implementation of the mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane).

The particle beam system400differs from the particle beam system300according to the third embodiment in that the functions provided by the first multipole field generator18and the third multipole field generator55are provided by a single multipole field generator referred to as first multipole field generator118. Accordingly, the mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane) is achieved by a two-stage direction-dependent focusing which is implemented by the first multipole field generated by the first multipole field generator118(second stage) and the second multipole field generated by the second multipole field generator50(first stage).

The first multipole field generator118has the same structure as the first multipole field generator18of the first to third embodiments illustrated inFIGS.7and8; however, the first multipole field generator118is operated differently. Accordingly, the first multipole field generated by the first multipole field generator118and the first multipole field generated by the first multipole field generator18are different.

For example, the first multipole field generator118is operated to generate the first multipole field so that the first multipole field provides both the second stage of the direction-dependent focusing according to the third embodiment and the energy-dispersive focusing/defocusing according to the first embodiment. That is, the first multipole field generator118is configured and operated so that the chromatic aberration of the focusing by the objective lens13in the first direction (x-axis) is reduced and that the chromatic aberration of the focusing by the objective lens13in the second direction (y-axis) is increased and is further configured to generate the first multipole field so that the charged particles of the beam3are focused according to a selectable third focusing power in the first plane (xz-plane) and that the charged particles of the beam3are focused according to a selectable fourth focusing power in the second plane (yz-plane), wherein the third focusing power is greater than the fourth focusing power. For example, the fourth focusing power is selected to be equal to the negative third focusing power.

Similar toFIGS.12and13,FIGS.16and17show the first and second plane in a rotating coordinate system rotating with the Larmor precession. That is, although illustrated in a flat plane, the x-axis and the y-axis rotate about the z-axis in dependence of the z-coordinate in accordance with the Larmor precession.

Effect of Beam Widening in Plane OL

Similar to the third embodiment, the particle beam system400according to the fourth embodiment generates a non-circular beam shape as illustrated inFIG.11, thus providing the same benefits.

In order to achieve a sufficient aspect ratio of the beam shape of the beam3in the objective lens13, a distance between the second multipole field generator50and the first multipole field generator118along the optical axis OA of the objective lens13is sufficiently large. In some embodiments, aspect ratios of the beam shape of the beam3in the objective lens13provided by the mechanism for independently manipulating the maximum illumination angles range from about 2 to 5. Therefore, the distance between the second multipole field generator50and the first multipole field generator118is about 5 cm to 20 cm.

Effect in Focal Plane FP, Beam Profile

Similar to the above embodiments, the particle beam system400according to the fourth embodiment generates a beam profile having a non-circular interaction profile35in the focal plane FP as illustrated inFIG.6, thus providing the same benefits. Also, the interaction length W1in the first direction D1obtained in the fourth embodiment is strongly reduced in comparison to the interaction length WN of a conventional particle beam system.

Adjusting the Orientation of the Beam Profile

The orientation of the beam profile of the beam3in the focal plane FP relative to the sample2can be adjusted by rotating the second multipole field generated by the second multipole field generator50and the first multipole field generated by the first multipole field generator118.

Improvement of First to Fourth Embodiments by Additional Octupole Field

The first to fourth embodiments described above each include the first multipole field generator18or118configured and operated to reduce chromatic aberration in the first direction D1at the cost of decreased resolution in the second direction D2. When excited in this manner, the first multipole field generator18or118also generates a 4-fold astigmatism C4configured (i.e., with an appropriate orientation and an appropriate sign) to reduce effects of spherical aberration in the first direction D1, even without explicit generation of an octupole field. However, the inventors found that an additional octupole field is beneficial for approaching the optimum state represented in equation (3) (described further below) in all modes of operation and is used for performing an optimization procedure OP6 (described further below).

Therefore, the first to fourth embodiments described above can be improved as follows. Further to the generating of the first multipole field acting on the beam3, the manipulating of the beam3(S3) further comprises: generating a fourth multipole field having an eight-pole component acting on the beam3. The fourth multipole field comprises at least one of a fourth electric multipole field having an eight-pole component and a fourth magnetic multipole field having an eight-pole component. That is, the fourth multipole field can comprise the fourth electric multipole field having the eight-pole component or can comprise the fourth magnetic multipole field having the eight-pole component or can comprise both. The fourth multipole field is generated so as to reduce effects of spherical aberration.

The fourth electric multipole field having the eight-pole component can be generated by a field generator having at least eight electrodes. For example, the fourth electric multipole field can be generated by the first multipole field generator18,118having eight electrodes23(seeFIG.8). The fourth magnetic multipole field having the eight-pole component can be generated by a field generator having at least eight coils. For example, the fourth magnetic multipole field can be generated by the first multipole field generator18,118having eight coils24(seeFIG.8).

The fourth multipole field can be rotated about the optical axis OA by appropriate excitation of the eight electrodes23and the eight coils24of the first multipole field generator18,118(seeFIG.8). In order to simultaneously generate the first multipole field having four electric poles and four magnetic poles and the fourth multipole field having the eight-pole component, the first multipole field generator18,118can be configured to individually control each of the electrodes23and the coils24, for example. However, other configurations are also conceivable. For example, the windings of the coils24can have multiple patterns, wherein one of the patterns provides windings for generating the four-pole component of the first magnetic multipole field and another one of the patterns provides windings for generating the eight-pole component of the fourth magnetic multipole field.

Alternatively, the fourth multipole field can be generated and rotated by a field generator having 16 electrodes and/or 16 coils, for example. Hereby the 16 electrodes (coils) can be arranged in a single plane perpendicular to the optical axis OA. Alternatively, a first set of 8 of the 16 electrodes (coils) can be arranged in a first plane perpendicular to the optical axis OA, and a second set of 8 of the 16 electrodes (coils) can be arranged in a second plane perpendicular to the optical axis OA, wherein the first and second plane are slightly separated along the optical axis OA. The first set of 8 electrodes (coils) and the second set of 8 electrodes (coils) can be rotated about the optical axis OA relative to each other, for example by approximately 22.5°.

When the first multipole field is rotated, the fourth multipole field is rotated the same. The rotation of the fourth multipole field can be achieved by appropriate excitation of the field generator generating the fourth multipole field. Alternatively, the field generator generating the fourth multipole field can be rotated.

A particle beam system500according to a fifth embodiment is described below with reference toFIGS.18and19. The particle beam system500according to the fifth embodiment differs from the particle beam system300according to the third embodiment in that the charged particle beam has a narrow energy bandwidth. Accordingly, chromatic aberration does not dominate the total aberration of the particle beam system, and thus, in the fifth embodiment, the first field generator18for compensating chromatic aberration according to the third embodiment is omitted. Further, the particle beam system500differs from the particle beam system300according to the third embodiment by further comprising a monochromator60.

Similar to the third embodiment, the second multipole field generator50and the third multipole field generator55provide the mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane). This is achieved by the same two-stage direction-dependent focusing described with reference to the third embodiment.

As an alternative mechanism for independently manipulating the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane), a (rotatable) aperture stop19having a non-circular aperture can be applied, as described above with reference to the second embodiment.

Similar toFIGS.12and13,FIGS.18and19show the first and second plane in a rotating coordinate system rotating with the Larmor precession. That is, although illustrated in a flat plane, the x-axis and the y-axis rotate about the z-axis in dependence of the z-coordinate in accordance with the Larmor precession.

The monochromator60is disposed upstream of the second multipole field generator50. The monochromator60is an energy filter configured to transmit charged particles having kinetic energies within a small predetermined energy bandwidth and to block charged particles having kinetic energies outside the predetermined energy bandwidth. For example, the monochromator60can be configured to reduce an energy width of the beam3provided by the particle source11to a value below 0.2 eV, such as below 0.1 eV, for example below 0.05 eV, about the predetermined kinetic energy En. The energy width is defined as full width at half maximum of a histogram of kinetic energies of the charged particles of the beam3. The monochromator60can be implemented by a Wien filter, for example. Alternatively, the monochromator60is an energy compressor. Using electric and/or magnetic AC-fields, particles with a kinetic energy lower than the nominal kinetic energy are accelerated, and particles with a kinetic energy higher than the nominal kinetic energy are decelerated. No particles are blocked and the beam current is preserved, but at a high technical expenditure.

Due to the small energy bandwidth of the charged particles emerging from the monochromator60, chromatic aberration of the objective lens13is not significant. However, in some systems, spherical aberration reduction can occur without the presence of a monochromator: If a high probe current is used, as in a beam inspection system, the illumination angle has to be increased to reach the desired probe current at the optimum resolution. Due to the cubic dependency of the spherical aberration on the illumination angle, the influence of the spherical aberration is strongly increased compared to the chromatic aberration with its linear dependency on the illumination angle. Consequently, in many cases, a total aberration of the particle beam system500is dominated by spherical aberration and an energy-dispersive focusing for chromatic aberration correction as provided in the first to fourth embodiment is not needed.

InFIGS.18and19(andFIGS.20and21), the fundamental ray6is illustrated as to emerge from a virtual source at the exit of the monochromator60. However, this is only a special case and not limiting. For example, the virtual source can be located inside the monochromator60.

Effect of Beam Widening

The inventors understood that, in an imaging system dominated by spherical aberration, adjusting a maximum illumination angle ϑxof a beam3in a first plane (xz-plane) to a first maximum illumination angle value and adjusting a maximum illumination angle ϑyof the beam3in a second plane (yz-plane) to a second maximum illumination angle value different from the first maximum illumination angle value manipulates the beam3so that the beam3has a beam profile having a non-circular interaction profile in the focal plane FP, the interaction profile in the focal plane FP having a shortest interaction length in a first direction (x-axis) and a largest line interaction length in a second direction (y-axis). Moreover, at the right choice of ϑxand ϑy, a smaller first interaction length W1can be reached than in the case of ϑx=ϑy. Consequently, the particle beam system500according to the fifth embodiment can provide similar benefits as the embodiments disclosed above.

Also, the interaction length W1in the first direction D1obtained in the fifth embodiment is strongly reduced in comparison to the interaction length WN of a conventional particle beam system.

Adjusting the Orientation of the Beam Profile

The orientation of the beam profile of the beam3in the focal plane FP relative to the sample2can be adjusted by rotating the second multipole field generated by the second multipole field generator50and the third multipole field generated by the third multipole field generator55.

Explanation of Effect in Focal Plane FP

A possible explanation of the effect of reduced interaction length of the beam3in the focal plane FP in the first direction (x-axis) by decreasing the second maximum illumination angle value (i.e., value of the maximum illumination angle ϑy) can be understood from Equations (1) below. Equations (1) describe a relation regarding spherical aberration between (i) a positional deviation (x, y) of the trajectory of a charged particle of the beam in the focal plane FP from the optical axis OA and (ii) the landing angles (αx, αy) of the charged particles, where Csdenotes a spherical aberration coefficient.

As can be understood from Equations (1), decreasing the (average) landing angle αyin the second plane (yz-plane) reduces the (average) deviation x of the trajectory from the optical axis in the first direction (x-axis), thereby reducing the interaction length in the focal plane FP in the first direction (x-axis). However, simultaneously, decreasing the (average) landing angle αyin the second plane (yz-plane) also has two impacts on the deviation y of the trajectory from the optical axis in the second direction (y-axis), namely a decreasing impact due to reduced effects of spherical aberration in accordance with Equations (1) and an increasing impact due to increased aperture diffraction. Overall, the diffraction effect might dominate, thereby increasing the interaction length in the second direction (y-axis).

A particle beam system600according to a sixth embodiment is described below with reference toFIGS.20to23.

Similar to the first embodiment, the particle beam system600comprises the particle source11, the objective lens13and the aperture stop17having the circular aperture16. The particle beam system600further comprises the monochromator60according to the fifth embodiment. The particle beam system600further comprises an octupole-field generator65configured to generate an octupole field.

The octupole field generated by the octupole-field generator65is used to reduce effects of spherical aberration of the objective lens13.

Implementations of the Octupole-Field Generator65

An exemplary implementation of the octupole-field generator65comprises sixteen electrodes for providing electric poles. By appropriate application of voltages to the sixteen electrodes, an electric octupole field of selectable strength and orientation about the optical axis OA of the objective lens13can be generated. The sixteen electrodes can be arranged in a single plane perpendicular to the optical axis OA. Alternatively, a first set of eight of the sixteen electrodes can be arranged in a first plane perpendicular to the optical axis OA, and a second set of eight of the sixteen electrodes can be arranged in a second plane perpendicular to the optical axis OA, wherein the first and second plane are slightly separated along the optical axis OA. The first set of eight electrodes and the second set of eight electrodes can be rotated about the optical axis OA relative to each other, for example by approximately 22.5°.

An exemplary implementation of the octupole-field generator65comprises sixteen coils for providing magnetic poles. By appropriate application of currents to the sixteen coils, a magnetic octupole field of selectable strength and orientation about the optical axis OA of the objective lens13can be generated. Alternatively, a first set of eight of the sixteen coils can be arranged in a first plane perpendicular to the optical axis OA, and a second set of eight of the sixteen coils can be arranged in a second plane perpendicular to the optical axis OA, wherein the first and second plane are slightly separated along the optical axis OA. The first set of eight coils and the second set of eight coils can be rotated about the optical axis OA relative to each other, for example by approximately 22.5°.

An exemplary implementation of the octupole-field generator65comprises eight electrodes23and eight coils24, as illustrated inFIG.8. The electrodes23generate an electric octupole-field; and the coils23generate a magnetic octupole-field. An octupole-force generated by the electric octupole-field is orientated like the electric octupole-field, while an octupole-force generated by the magnetic octupole-field is rotated by 22.5° against the magnetic (or electric) octupole-field. Therefore, even the first multipole field generator18of the first embodiment is capable of generating a rotatable octupole-force by superimposing an electric octupole field and a magnetic octupole field as implemented with reference toFIG.8.

Effect in Focal Plane FP, Beam Profile

Referring toFIGS.22A-22D and23, the effect of the octupole field on the beam3is described.FIGS.22A-22Dshow a schematic illustration of a resulting beam profile36of the beam3in the focal plane FP. The beam profile36of the beam3in the focal plane FP is schematically illustrated by three isolines and the center O of the intensity distribution of the beam3, previously introduced with reference toFIGS.5A and5B. However, the beam profile36of the beam3illustrated inFIGS.22A-22Ddiffers considerably from the beam profile34illustrated inFIGS.5A and5B. In the example illustrated inFIGS.22A-22D, the isolines have a shape of a cross having rounded vertices. The isolines are centered on the center O of the intensity distribution of the beam3in the focal plane FP.

Similar toFIGS.5A and5B, the beam profile36of the beam3in the focal plane FP will be characterized by the direction-specific interaction length. Accordingly,FIG.22Ashows a first straight L1line perpendicular to a first direction D1and a second straight line L2perpendicular to the first direction D1. The first straight line L1defines a first half-plane HP1. The first half-plane HP1is located in the focal plane FP (focal plane FP is not illustrated inFIGS.22A-22D) and contains 25% of a total intensity of the beam3in the focal plane FP. Similarly, the second straight line L2defines a second half-plane HP2. The second half-plane HP2is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. The first half-plane HP1and the second half-plane HP2do not overlap. The first interaction length W1is the distance, measured along the first direction D1, between the first straight line L1and the second straight line L2.

Similarly,FIG.22Bshows a third straight L3line perpendicular to a second direction D2and a fourth straight line L4perpendicular to the second direction D2. The third straight line L3defines a third half-plane HP3. The third half-plane HP3is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. Similarly, the fourth straight line L4defines a fourth half-plane HP4. The fourth half-plane HP4is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. The third half-plane HP3and the fourth half-plane HP4do not overlap. The second interaction length W2is the distance, measured along the second direction D2, between the third straight line L3and the fourth straight line L4.

Similarly,FIG.22Cshows a fifth straight L5line perpendicular to a third direction D3and a sixth straight line L6perpendicular to the third direction D3. The fifth straight L5defines a fifth half-plane HP5. The fifth half-plane HP5is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. Similarly, the sixth straight line L6defines a sixth half-plane HP6. The sixth half-plane HP6is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. The fifth half-plane HP5and the sixth half-plane HP6do not overlap. The third interaction length W3is the distance, measured along the third direction D3, between the fifth straight line L5and the sixth straight line L6.

Similarly,FIG.22Dshows a seventh straight L7line perpendicular to a fourth direction D4and an eight straight line L8perpendicular to the fourth direction D4. The seventh straight L7defines a seventh half-plane HP7. The seventh half-plane HP7is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. Similarly, the eighth straight line L8defines an eighth half-plane HP8. The eighth half-plane HP8is located in the focal plane FP and contains 25% of the total intensity of the beam3in the focal plane FP. The seventh half-plane HP7and the eighth half-plane HP8do not overlap. The fourth interaction length W4is a distance, measured along the fourth direction D4, between the seventh straight line L7and the eighth straight line L8.

The first direction D1, the second direction D2, the third direction D3and the fourth direction D4are mutually different directions.

The beam3is manipulated by the octupole field so that the first interaction length W1is significantly smaller than the second interaction length W2and that the third interaction length W3is significantly smaller than the fourth interaction length W4. For example, the octupole field is generated so that a ratio of the first interaction length W1to the second interaction length W2amounts to at most 1:1.2 and that a ratio of the third interaction length W3to the fourth interaction length W4amounts to at most 1:1.2. Optionally, the ratio of the first interaction length W1to the second interaction length W2amounts to at most 1:1.5 or at most 1:2.0. Optionally, the ratio of the third interaction length W3to the fourth interaction length W4amounts to at most 1:1.5 or at most 1:2.0.

As illustrated inFIGS.22A-22D, the octupole field can be generated so that the first direction D1and the third direction D3are perpendicular to each other while the second direction D2and the fourth direction D4are perpendicular to each other.

FIG.23shows a schematic illustration of an interaction profile37of the beam3having the beam profile36illustrated inFIGS.22A-22D. Please note thatFIGS.22A-22D and23have different scales, thus, in comparison toFIGS.22A-22D, the same interaction length appears longer inFIG.23. Similar toFIG.6, the interaction profile37represents the direction-specific interaction lengths for all directions of the focal plane FP. For the beam profile36illustrated inFIGS.22A-22D, this results in an interaction profile37having a shape of a lying cross (i.e., a cross rotated by 45°) having rounded vertices when the lines representing the interaction lengths are centered on a common center O.

As illustrated inFIGS.22A-22D and23, the interaction length W1for the first direction D1is shorter than the interaction length W2for the second direction D2; and the interaction length W3for the third direction D3is shorter than the interaction length W4for the fourth direction D4. Consequently, when using the beam3as a scanning microscope probe, in a resulting image, the maximum resolution in the first direction D1is higher than the maximum resolution in the second direction D2; and the maximum resolution in the third direction D3is higher than the maximum resolution in the fourth direction D4; while the resolutions in the first direction D1and the third direction D3are the same, and the resolutions in the second direction D2and the fourth direction D4are the same. Overall, the beam profile36is such that high resolution can be obtained simultaneously in two directions, i.e., in the first direction D1and the third direction D3, at the cost of low resolution in two other directions, i.e., in the second direction D2and the fourth direction D4.

Also, the interaction length W1in the first direction D1and the interaction length W3in the third direction D3obtained in the present embodiment are strongly reduced in comparison to the interaction length WN of a conventional particle beam system. Therefore, the present embodiment simultaneously provides a strongly improved resolution in the first direction D1and in the third direction D3in comparison to the conventional particle beam system having uniform resolution in all directions.

Mode of Operation: Cross-Shaped Interaction Profile

In order to obtain the above-described effect, the octupole field is generated in a specific way described below. The influence of the octupole field sums up in the third order part of the aberration deviation in the beam profile as

In Equations (2) above, the aberration coefficient C4for a 4-fold astigmatism describes the strength of the octupole field, CSis the spherical aberration coefficient, and αxand αydenote landing angles along the x-axis and along the y-axis.

By adjusting the strength of the octupole field so that C4corresponds to CS/3, Equation (2) becomes:

Accordingly, in comparison to the situation without octupole field, the aberration is strongly reduced on the diagonals of the x-y-coordinate system, while the aberration on the x-axes and y-axes is slightly increased. However, in the generation of the interaction profile the diagonals dominate the averaging process. This results in a cross-shaped beam profile as illustrated inFIGS.22A-22D, and a rotated cross-shaped interaction profile illustrated inFIG.23.

Mode of Operation: Square-Like Interaction Profile

By adjusting the strength of the octupole field so that C4corresponds to approximately 0.17·CS, the beam is manipulated into an approximately square-like beam profile38in the focal plane FP, which is schematically illustrated inFIG.24. When machining a sample with a charged particle beam by milling, etching, or depositing, the beam profile is printed to the sample surface. A square-like beam profile can be desirable for writing the corners of rectangular structures.

Adjusting the Orientation of the Beam Profile

The orientation of the beam profile of the beam3in the focal plane FP relative to the sample2can be adjusted by rotating the octupole field generated by octupole-field generator65.

Optimizing the Beam Profile

In the particle beam systems100to600described herein, multipole fields are generated und used for manipulating the beam profile of the beam3. As explained with reference toFIGS.5and22A-22D, the objective of the manipulating is to achieve a short first interaction length W1in the first direction which is shorter than in a conventional particle beam system. The particle beam systems100to600described herein provide multiple different parameters that are used to obtain a short first interaction length W1in the first direction. These parameters can be the parameters defining an excitation of the multipole field generators (first multipole field generator18,118; second multipole field generator50; third multipole field generator55; octupole-field generator65), for example. Further, these parameters can be the maximum illumination angles, e.g., the maximum illumination angle ϑxof the beam3in the first plane (xz-plane) and the maximum illumination angle ϑyof the beam3in the second plane (yz-plane). For example, these parameters are the voltages applied to electrodes of the multipole field generators and the currents applied to coils of the multipole field generators.

Conventional particle beam systems are optimized to obtain high resolution in all directions of the focal plane. In terms of the beam profile illustrated inFIGS.5and22A-22D, conventional particle beam systems are optimized to have equal interaction length in all directions of the focal plane. In contrast to this approach, the embodiments of the present disclosure are optimized to obtain high resolution in a single direction or two directions, thereby obtaining a higher resolution in this selected direction (these two selected directions) compared to the conventional particle beam systems. This effect comes at the price of worse resolution in other directions compared to the conventional particle beam systems.

Conventional particle beam systems use a stigmator (an electric or magnetic field generator capable of generating a weak, rotatable quadrupole field) to compensate for residual astigmatism which is generated by properties of optical elements, i.e., by mechanical tolerances and/or material inhomogeneities of an objective lens. The particle beam systems100to600described herein can also comprise such a stigmator for the same purpose. Alternatively, any of the multipole field generators (first multipole field generator18,118; second multipole field generator50; third multipole field generator55; octupole-field generator65) can be used as a stigmator to generate a weak, rotatable, and electric or magnetic quadrupole field in addition to its primary purpose. This weak quadrupole field weakly focuses the charged particle beam3in a fifth direction D5and weakly defocuses in a sixth direction D6, which is perpendicular to the fifth direction D5, and is configured to compensate for the residual astigmatism of the particle column10. While the target orientations of the first direction D1, the second direction D2, the third direction D3, and the fourth direction D4of the manipulated beam profile are selected to different values, the fifth direction D5and the sixth direction D6remain fixed.

According to a first optimization routine, the parameters are optimized to obtain a shortest interaction length in one single direction.FIGS.5A and5Billustrate an example of the beam profile34obtained by the first optimization routine, andFIG.6illustrates the resulting interaction profile35of the beam profile34. As illustrated inFIG.6, the shortest interaction length is the interaction length W1for the first direction D1. That is, all available parameters are determined so that a shortest interaction length is obtained for one single direction in the focal plane FP.

According to a second optimization routine, the parameters are optimized to obtain short interaction lengths in two directions.FIGS.22A-22Dillustrate an example of the beam profile36obtained by the second optimization routine, andFIG.23illustrates the resulting interaction profile37of the beam profile36. As illustrated inFIG.23, the short(est) interaction lengths are the first interaction length W1for the first direction D1and the third interaction length W3for the third direction D3. That is, all available parameters are determined so that two short interaction lengths for two different directions (i.e., one for each direction) are obtained.

The optimization can be performed by standard routines. For example, optimized values for the parameters can be obtained by trial and error, experiment, simulation or a combination thereof. The beam profile can be obtained by measuring the intensity distribution of the beam3in the focal plane FP and evaluating the measured data.

Depending on the particular configuration of the charged particle beam system and the intended application, the optimization can include any of the following steps that can be selectively and iteratively applied:(OP1) Align the beam3to the optical axes of all optical elements by electrical or mechanical alignment.(OP2) Adjust the multipole field generator18,118to compensate the chromatic aberration in the first direction D1.(OP3) Adapt the maximum illumination angle ϑxso that the superposition of diffraction and aberrations in the first direction D1leads to a minimum interaction length W1. If direction-dependent beam widening is not available, the maximum illumination ϑyis adapted to the same value.(OP4) If direction-dependent beam widening is available, adapt the maximum illumination angle ϑy(independently from ϑx) so that the superposition of diffraction and aberrations in the second direction D2leads to a minimum interaction length W2.(OP5) If direction-dependent beam widening is available, reduce the maximum illumination angle ϑy(independently from ϑx) so that the diffraction in the second direction D2leads to an interaction length W2that has a maximum allowed value, thereby reducing the interaction length W1in the first direction D1.(OP6) Adapt the 4-fold astigmatism C4by exciting an octupole field with the octupole field generator65or the multipole field generator18,118so that the interaction length W1in the first direction D1adopts a minimum.(OP7) Fine adjust defocus and astigmatism by changing the excitation of objective lens13(and stigmator if present) so that the resolution in the first direction D1and the resolution in the direction perpendicular to the first direction D1are the best.

Hereinabove, an influence of defocus and astigmatism was not discussed, because it makes explanations more complicated and figures less clear. Nevertheless, this influence can be numerically simulated or experimentally measured, so that the optimization procedure OP7 is feasible.

At a low landing energy, the chromatic aberration usually dominates the aberrations of a particle beam system. The chromatic aberration, at least in the first direction D1, should be compensated, like shown in the exemplary particle beam systems100,200,300, and400. If no mechanism for generating direction-dependent beam widening is provided, like in the exemplary particle beam system100, the optimization procedures OP1, OP2, OP3, and OP7 should be iteratively applied.

If a mechanism for generating a rotatable octupole field are provided, like by an additional octupole field generator65or a corresponding excitation mechanism for the multipole field generator18, the effects of spherical aberration in the first direction D1can be reduced by introducing a 4-fold astigmatism C4, as discussed with equation (2). Here the optimization procedures OP1, OP2, OP3, OP6, and OP7 should be iteratively applied.

If one is interested in a maximum ratio W1/W2, a mechanism for a rotatable, direction-dependent beam widening can be provided, like shown in the exemplary particle beam systems200,300, and400. Here the optimization procedures OP1, OP2, OP3, OP4, and OP7 should be iteratively applied.

If the interaction length W1in the first direction D1should be further reduced, and a certain interaction length W2in the second direction D2is allowed, the effects of spherical aberration in the first direction D1can be reduced by changing the beam widening, as discussed with equation (1). Here the optimization procedures OP1, OP2, OP3, OP5, and OP7 should be iteratively applied.

If the interaction length W1in the first direction D1should be further reduced, and one is interested in a maximum ratio W1/W2, a mechanism for generating a rotatable octupole field can be provided, like by an additional octupole field generator65or a corresponding excitation mechanism for the multipole field generator18,118. Thereby the effects of spherical aberration in the first direction D1can be reduced by introducing a 4-fold astigmatism C4, as discussed with equation (2). Here the optimization procedures OP1, OP2, OP3, OP4, OP6, and OP7 should be iteratively applied.

It should be noted that the multipole field generator18, when excited for a chromatic aberration compensation in the first direction D1, already produces a 4-fold astigmatism C4with the right orientation and sign for the reduction of the effects of spherical aberration in the first direction D1, without any octupole field present. However, to come near to the optimum state shown in equation (3) in all modes of operation, an additional octupole field is used to perform the optimization procedure OP6.

Additionally, if the direction-dependent beam widening is used according to optimization procedure OP4, there is already a reduction of the effects of spherical aberration in the first direction D1as discussed with equation (1). Switching optimization procedures from OP4 to OP5 can further reduce the effects of spherical aberration in the first direction D1.

If using a high landing energy, a monochromator, or a high beam current (like in a beam inspection system), the spherical aberration usually dominates the aberrations of a particle beam system. Here a compensation of the chromatic aberration is not needed.

In such a system, the effects of spherical aberration can be reduced in one direction by direction-dependent beam widening, like shown in the exemplary particle beam system500. Here the optimization procedures OP1, OP3, OP5, and OP7 should be iteratively applied.

Alternatively, the effects of spherical aberration can be reduced in two directions by 4-fold astigmatism, like shown in the exemplary particle beam system600. Here the optimization procedures OP1, OP3, OP6, and OP7 should be iteratively applied.

It depends on the type of application which of these two methods for the reduction of the effects of spherical aberration should be chosen, but the optimized resolution in two directions at the same time can be desirable for the use of the 4-fold astigmatism.

Alignment of Multipole Fields

In the particle beam systems described herein, multiple electric and/or magnetic fields are generated. Proper alignment of these fields is thus used to obtain the desired improvements.

Due to mechanical tolerances and inhomogeneity of magnetic material properties, the center of an electric multipole field, the center of a magnetic multipole field and the beam position might deviate from each other. In order to align these, electric and magnetic dipole fields overlapping with the multipole fields can be generated to shift the multipole fields.

Shared supplies for oppositely arranged electrodes/coils, i.e., electrodes and coils arranged opposite to each other across the optical axis of the respective field generator (where the polarities of opposing coils are chosen so that their dipole fields in the center of the multipole practically cancel out), improve stability of the generated fields, but have the drawback of preventing dipole fields for alignment. For the magnetic field, this issue can be addressed by using additional coils, referred to as alignment coils, having a small maximum excitation (where the polarities of opposing alignment coils are chosen so that their dipole fields in the center of the multipole add up). The alignment coils are driven with a small maximum current, just as high to enable the alignment of the magnetic multipole field. Consequently, deflection induced by current noise and drift in these coils will be also small, so that the overall system stability is maintained. A smaller maximum current or less alignment coil windings lead to a smaller deflection by the noise and drift of the current sources but reduce the maximum shift of the center of the magnetic multipole field and, therefore, the allowable mechanical tolerance range.

There are two ways to generate the electric dipole field for aligning the electric multipole field when using shared voltage supplies, here presented for the example of eight electrodes, but the principles of the solution can be applied to any other number of electrodes. The first way is to insert a floating voltage source with a small output range in series between every electrode and its corresponding base voltage supply output. These floating voltage sources provide the electrodes with small alignment voltages. Applying opposite alignment voltages, i.e., alignment voltages of the same magnitude but different sign, to oppositely arranged electrodes produces a weak dipole field, which is sufficient for the alignment of the electric multipole field but does not generate too much deflection by noise and drift of the alignment voltage sources. For this solution eight additional floating voltage sources are used.

The second way is to use a resistor network to mix the signals on the electrodes to produce a weak dipole field and a strong multipole field. Assume that four voltage sources with complementary outputs and identical output ranges are used, the outputs QC+ and QC− are available for a first quadrupole field, outputs QS+ and QS− for a second quadrupole field, outputs DX+ and DX− for the first dipole field, and outputs DY+ and DY− for the second dipole field. When numbering the electrodes counterclockwise from U1 to U8, the electrodes are connected to these outputs by resistors with values listed in Table 1. The base value R may be chosen appropriately.

Many resistor network configurations are possible, but the configuration listed in Table 1 results in the minimum number of resistors. The strength of the dipole field can be changed by scaling the numbers 10 and 14 in the table by the same factor. Larger values lead to a smaller deflection by the noise and drift of the voltage supplies but reduce the maximum shift of the center of the electric quadrupole field and, therefore, the allowable mechanical tolerance range.

The resistor network does not necessarily need to be connected directly to the electrodes. Instead, the output range of the original amplifiers may be reduced, and additional amplifiers may be inserted into the lines between the resistor network and the electrodes. In this way the resistors can work at a lower voltage, while simple amplifiers usually do not introduce too much noise or drift; but the optimum solution depends on the layout of the complete imaging system.

Alternatively, external deflection elements can be inserted in front of one or more multipole fields. With these deflection elements, the central trajectory of the charged particle beam3can be manipulated to pass the multipole fields at their real centers. When using a double deflection element, even the direction can be adjusted, so that the real axis of the multipole field is used as an optical axis over its whole length. This can be desirable for a multipole field which is long compared to its bore diameter. Since only weak deflection elements are involved, noise and drift are not a problem.

If the center of the magnetic multipole does not coincide with the center of the electric multipole due to magnetic inhomogeneities, external deflection elements are not sufficient. Weak alignment coils in the multipole can be used to shift the axis of the magnetic multipole field to the axis of the electric multipole field, like described earlier.

However, even if the centers of all electric and magnetic quadrupole fields are shifted to the position of the beam, or the position of the beam to the respective center of each quadrupole field, a small residual image shift may still occur when the orientation of the probe shape is changed. Therefore, when acquiring a series of images, every image can be moved back to its non-shifted position. This non-shifted position can be determined by a cross-correlation between subsequent images or between every image and a reference image, but noise can deteriorate the result. To reduce the influence of noise, the image shifts determined by the cross-correlation can be fitted by the formula

where X(i), Y(i) is the image shift of the image with index i within the image series relative to the reference image with index n, A is the residual shift amplitude induced by the combined action of all multipole fields, β is an angular step size in the orientation of the beam profile, γ is the residual shift orientation induced by the combined action of all multipole fields, BX, BY are drift velocities along the x-axis and y-axis, respectively, and T(i) is the starting time of the acquisition of the image with index i. Since β, n, and T(i) are known, the parameters A, γ, BX, and BY can be used to fit the determined image shifts. Hence, all shift data generated from the application of the cross-correlation function is now projected to only these four parameters, so that a strong averaging is done, and the influence of noise is effectively reduced.

First Application: Improved Image Resolution in any Direction

FIG.25shows a flowchart illustrating a first method of operating the particle beam systems described herein. The objective of the first method is to obtain an image of a sample2having improved resolution in any direction (i.e., high resolution in all directions). Steps S2to S4of the first method do not have to be performed in the order indicated by arrows of the flowchart. Instead, the steps S2to S4of the first method can be performed in any order. For example, the steps S2to S4of the first method can be performed simultaneously.

According to step S1, the first method comprises generating a beam3of charged particles (e.g., electrons). For example, the step S1can be performed by the particle source11of the particle beam systems described herein.

After step S1, according to step S2, the first method further comprises focusing the beam3into a focal plane FP. For example, the step S2can be performed by the objective lens13of the particle beam systems described herein.

According to step S3, the first method further comprises manipulating the beam3so that the beam3has a beam profile in the focal plane FP having a non-circular interaction profile in the focal plane FP. For example, according to the first embodiment, the step S3can be performed by the first multipole field generator18. Further, according to the second embodiment, the step S3can be performed by the first multipole field generator18and the aperture stop19having the non-circular aperture. Further, according to the third embodiment, the step S3can be performed by the first multipole field generator18, the second multipole field generator50and the third multipole field generator55. Further, according to the fourth embodiment, the step S3can be performed by the first multipole field generator118and the second multipole field generator50. Further, according to the fifth embodiment, the step S3can be performed by the second multipole field generator50and the third multipole field generator55. Further, according to the sixth embodiment, the step S3can be performed by the octupole field generator65. Step S3causes the beam3to have a shortest interaction length along the first direction and a largest interaction length along the second direction (seeFIG.6). In some cases, step S3causes the beam3to have two (or more) short interaction lengths (e.g., short interaction lengths W1and W3along the first and third directions, respectively, seeFIG.23) at the cost of the beam3having two (or more) long interaction lengths (e.g., long interaction lengths W2and W4along the second and fourth directions, respectively, seeFIG.23).

According to step S4, the first method further comprises adjusting an orientation of the beam profile in the focal plane FP relative to the sample2(located in the focal plane FP) to a target orientation. For example, the step S4can be performed by rotating the sample2. Rotating the sample2can be performed by controlling the sample stage4to rotate the sample2, for example. Further, according to the first embodiment, the step S4can be performed by rotating the first multipole field generated by the first multipole field generator18. Further, according to the second embodiment, the step S4can be performed by rotating the first multipole field generated by the first multipole field generator18and rotating the aperture stop19having the non-circular aperture. Further, according to the third embodiment, the step S4can be performed by rotating the first multipole field generated by the first multipole field generator18, rotating the second multipole field generated by the second multipole field generator50and rotating the third multipole field generated by the third multipole field generator55. Further, according to the fourth embodiment, the step S4can be performed by rotating the multipole field generated by the first multipole field generator118and rotating the second multipole field generated by the second multipole field generator50. Further, according to the fifth embodiment, the step S4can be performed by rotating the second multipole field generated by the second multipole field generator50and rotating the third multipole field generated by the third multipole field generator55. Further, according to the sixth embodiment, the step S4can be performed by rotating the octupole field generated by the octupole field generator65. Step S4causes the first direction (i.e., direction of highest resolution) to be orientated along a selected target orientation with respect to the sample2.

According to step S5, the first method further comprises recording an image of the sample2located in the focal plane FP using the manipulated beam3(i.e., the beam having the beam profile manipulated according to step S3) having the adjusted orientation (i.e., adjusted according to step S4). The image recorded in step S5has a highest resolution along the first direction (and the third direction) because the shortest interaction length of the interaction profile of the beam3in the focal plane FP is orientated along the first direction (and the third direction). The image recorded in step S5has a worst resolution along the second direction (and the fourth direction) because the largest interaction length of the interaction profile of the beam3in the focal plane FP is orientated along the second direction (and the fourth direction).

For example, the recording of the image according to step S5can comprise: maintaining the adjusted orientation while directing the manipulated beam3to a plurality of locations of the sample2; detecting interaction products of an interaction of the manipulated beam3with the sample2during the directing of the manipulated beam3to the plurality of locations of the sample2; and generating the image based on the detected interaction products. Maintaining the adjusted orientation mechanism to not change the orientation of the beam profile of the beam3relative to the sample2. The detecting of the interaction products can be performed by the detection system40of the particle beam systems described herein. The generating of the image can be performed by the controller30of the particle beam systems described herein.

According to step S6, the first method further comprises determining whether another image is to be recorded. For example, the step S6can be performed by the controller30of the particle beam systems described herein. If the determination in step S6is to record another image (yes at step S6), step S7is performed next. If the determination in step S6is to not record another image (no at step S6), step S8is performed next.

According to step S7, the first method further comprises changing the target orientation. For example, the step S7can be performed by the controller30of the particle beam systems described herein. Subsequent to step S7, steps S4to S7are repeated until the determination in step S6is to not record another image. As a result, a plurality of images of the sample2are recorded at different orientations of the beam profile of the beam3in the focal plane FP relative to the sample2. Consequently, the direction of highest resolution differs among the recorded images. When using the beam profile illustrated inFIGS.5A and5B, where the beam profile exhibits only one direction providing maximum resolution (i.e., the first direction), many images are recorded to cover a near 180° rotation of the target orientation. In contrast, when using the beam profile illustrated inFIGS.22A-22D, where the beam profile exhibits two directions providing maximum resolution, less images are recorded to cover a near 90° rotation of the target orientation.

According to step S8, the first method further comprises calculating a synthesized image of the sample2based on the recorded images (i.e., the images recorded in step S5). For example, the step S8can be performed by the controller30of the particle beam systems described herein.

A variety of suitable algorithms can be used for the calculating of the synthesized image of the sample2based on the recorded images. Suitable algorithms can use the fact that the resolution of each of the recorded images is not uniform but direction dependent. That is, each of the recorded images exhibits a highest resolution along the first direction defined by the interaction profile of the beam3in the focal plane FP during the recording of the respective image and exhibits a worst resolution along the second direction defined by the beam profile of the beam3in the focal plane FP during the recording of the respective image. When using the beam profile illustrated inFIGS.22A-22D, each of the recorded images exhibits a highest resolution along the first and third direction defined by the beam profile of the beam3in the focal plane FP during the recording of the respective image and exhibits a worst resolution along the second and fourth direction defined by the beam profile of the beam3in the focal plane FP during the recording of the respective image. Due to steps S7and S4, the orientation of the beam profile of the beam3in the focal plane FP is changed between subsequent recordings in step S5. Consequently, the directions of highest resolution in the recorded images are different from each other. However, by appropriate combination of the recorded images, a synthesized image having improved resolution in all directions can be obtained. Hereinafter, two exemplary algorithms are described.

Non-Uniform Weighting in Dependence of the Target Orientations

By appropriately weighting high-resolution contributions of the recorded images higher than low-resolution contributions of the recorded images, the synthesized image can exhibit high resolution in any direction.

According to a specific example, calculating the synthesized image comprises: weighting the recorded images using non-uniform weight distributions, wherein orientations of the weight distributions are selected to correspond to the target orientations; and merging the weighted images.

According to this example, the recorded images exhibiting a direction-dependent maximum resolution are weighted using non-uniform weight distributions and the weighted images are merged (e.g., averaged), thereby generating the synthesized image. For example, each of the recorded images is weighted by (e.g., multiplied with) one of the non-uniform weight distributions, and the weighted images are merged. The expression non-uniform weight distribution mechanism that different pixels/areas of an image are weighted by different strengths (e.g., different values). Exemplifying the weight distribution by a scalar field, a non-uniform scalar field would be characterized by including different values.

Further, each of the non-uniform weight distributions is associated with a direction/orientation which is characteristic for the particular weight distribution. Accordingly, based on the orientation of the weight distribution, different pixels/areas of an image are weighted with different strengths. By selecting the orientations of the non-uniform weight distributions to correspond to the target orientations, i.e., the directions of maximum resolution in the recorded images, the non-uniform weight distributions allow to increase a contribution of pixels/areas exhibiting high maximum resolution of each image to the synthesized image while a contribution of pixels/areas exhibiting low maximum resolution of each image is decreased. Thus, high maximum resolution pixels/areas of the recorded images contribute more to the synthesized image than low maximum resolution pixels/areas of the recorded images, thereby increasing the maximum resolution in any direction.

According to another specific example, the calculating of the synthesized image comprises weighting the recorded images using non-uniform weight distributions wi({right arrow over (k)}) and merging the weighted images. According to a specific example, the weight distributions wi({right arrow over (k)}) fulfil wi({right arrow over (k)}i,1)>wi({right arrow over (k)}i,2), wherein i represents an index identifying an i-th one of the recorded images and ranges over all of the recorded images, wi({right arrow over (k)}i,1) represents a weight for a spatial-frequency domain component of the i-th recorded image at spatial-frequency {right arrow over (k)}i,1, wi({right arrow over (k)}i,2) represents the weight for the spatial-frequency domain component of the i-th recorded image at spatial-frequency {right arrow over (k)}i,2, {right arrow over (k)}i,1represents a spatial-frequency of magnitude K in a spatial-frequency domain direction corresponding to the first direction, and {right arrow over (k)}i,2represents a spatial-frequency of magnitude K in a spatial-frequency domain direction corresponding to the second direction.

When using the beam profile illustrated inFIGS.22A-22D, according to a specific example, the weight distributions wi({right arrow over (k)}) further fulfil wi({right arrow over (k)}i,3)>wi({right arrow over (k)}i,4), wherein wi({right arrow over (k)}i,3) represents a weight for a spatial-frequency domain component of the i-th recorded image at spatial-frequency {right arrow over (k)}i,3, wi({right arrow over (k)}i,4) represents the weight for the spatial-frequency domain component of the i-th recorded image at spatial-frequency {right arrow over (k)}i,4, {right arrow over (k)}i,3represents a spatial-frequency of magnitude K in a spatial-frequency domain direction corresponding to the third direction, and {right arrow over (k)}i,4represents a spatial-frequency of magnitude K in a spatial-frequency domain direction corresponding to the fourth direction.

According to another specific example, one non-uniform weight distribution is used for weighting all of the recorded images, but the orientation of the non-uniform weight distribution is changed for each weighting to correspond to the target orientation with which the respective image was recorded. In other words, for weighting a particular one of the recorded images, the non-uniform weight distribution is rotated to match the target orientation with which the particular image was recorded, and then the rotated non-uniform weight distribution is applied to the particular recorded image to form a weighted image.

Deconvolution Algorithms

Generating the synthesized image based on the recorded images can involve algorithms of joint deconvolution, in which the recorded images are combined considering orientation-dependent non-uniform point spread functions (PSF). Examples of such algorithms include weighted averaging using rotation-specific point spread functions and Richardson-Lucy deconvolution.

According to a specific example, calculating the synthesized image comprises: convolving the recorded images using non-uniform point spread functions, wherein orientations of the point spread functions are selected to correspond to the target orientations; and merging the convolved images.

According to this example, the recorded images exhibiting a direction-dependent maximum resolution are convolved using non-uniform point spread functions and the convolved images are merged (e.g., averaged), thereby generating the synthesized image. For example, each of the recorded images is convolved by one of the non-uniform point spread functions, and the convolved images are merged. The expression non-uniform point spread functions mechanism that the point spread functions differ from each other. For example, the point spread functions can correspond to a same point spread function rotated to different directions corresponding to the target orientations.

Further, each of the non-uniform point spread functions is associated with a direction/orientation which is characteristic for the particular point spread function. Accordingly, based on the orientation of the point spread function, different pixels/areas of an image are weighted with different strengths. By selecting the orientations of the non-uniform point spread functions to correspond to the target orientations, i.e., the directions of maximum resolution in the recorded images, the non-uniform point spread functions allow to increase a contribution of pixels/areas exhibiting high maximum resolution of each image to the synthesized image while a contribution of pixels/areas exhibiting low maximum resolution of each image is decreased. Thus, high maximum resolution pixels/areas of the recorded images contribute more to the synthesized image than low maximum resolution pixels/areas of the recorded images, thereby increasing the maximum resolution in any direction.

Note that convolving with a given point spread function is equivalent to deconvolving with an inverse of the given point spread function. In other words, instead of convolving the recorded images, the recorded images can be deconvolved with another point spread function and the deconvolved images can be merged.

Second Application: Improved Image Resolution in One Direction

FIG.26shows a flowchart illustrating a second method of operating the particle beam systems described herein. The objective of the second method is to obtain an image of a sample2having improved resolution in a single direction of particular interest. For example, for analyzing distances of a line structure on a sample comprising a plurality of separated parallel lines, high resolution of an image of the sample along a direction perpendicular to the direction of the lines is of particular interest whereas high resolution in a direction parallel to the direction of the lines is not of particular interest. In such use cases, high resolution in a single direction is more important than mediocre resolution in all directions.

The second method comprises the steps S1to S5of the first method described with reference toFIG.25. Reference is made to the corresponding description. Steps S2to S4of the second method do not have to be performed in the order indicated by arrows of the flowchart. Instead, the steps S2to S4of the second method can be performed in any order, provided that step S9is performed prior to step S4. For example, the steps S2to S4of the second method can be performed simultaneously.

According to step S9to be performed before step S4, the second method further comprises selecting the target orientation based on an orientation of a structure on the sample. For example, the target orientation can be selected so that, in step S4, the first direction (i.e., the direction along which the interaction profile of the beam3in the focal plane FP has the shortest interaction length) is orientated perpendicular to a line structure on the sample2. For example, the step S9can be performed by the controller30of the particle beam systems described herein. By selecting the target orientation, the direction of highest resolution of the image to be recorded in step S5can be selected as desired. For example, in a chip fabrication process, an orientation of a wafer in the fabrication process and an orientation of structures on the wafers are held in a controller controlling the chip fabrication process. Consequently, the target orientation may be defined based on a specification of the chip fabrication process. For example, in an experimental setup, an orientation of structures to be analyzed in more detail based on an image obtained by the second method can be obtained based on an image of the structures obtained by conventional approaches.

When the interaction pattern of the beam is manipulated to provide improved resolution in two different directions D1and D3, as in the example of the sixth embodiment, these two directions D1and D3can be adapted to the structure on the sample2. For example, when the structure on the sample2predominantly has lines arranged in a first structure direction (e.g., a horizontal direction) and lines arranged in a second structure direction (e.g., a vertical direction), these lines can be resolved best when the direction D1is perpendicular to the first structure orientation (e.g., the direction D1is perpendicular to the horizontal direction) and the direction D3is perpendicular to the second structure direction (e.g., the direction D3is perpendicular to the vertical direction). This benefit can be achieved by, in step S3, manipulating the beam so that the first direction D1and the third direction D3of the beam interaction pattern are arranged relative to each other in a pattern corresponding to the arrangement pattern of the structure directions and performing step S9.

Subsequent to step S9, according to the step S4, an orientation of the beam profile in the focal plane FP relative to the sample2is adjusted to the target orientation selected in step S9.

Subsequent to step S4, according to the step S5, an image of the sample2located in the focal plane FP is recorded using the manipulated beam3(i.e., the beam having the beam profile manipulated according to step S3) having the adjusted orientation (i.e., adjusted according to step S4).

Therefore, an image of the sample2having improved resolution in a single direction which is of particular interest can be recorded by the second method.

In some implementations, the controller30can include one or more data processors for processing data, one or more storage devices for storing data, and/or one or more computer programs including instructions that when executed by the controller30cause the controller30to carry out the methods described herein.

In some implementations, the controller30can include digital electronic circuitry, computer hardware, firmware, software, or any combination of the above. The features related to processing of data can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device, for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described systems and methods by operating on input data and generating output. Alternatively or additionally, the program instructions can be encoded on a propagated signal that is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a programmable processor. In some implementations, the operations associated with processing of data described herein can be performed by one or more programmable processors executing one or more computer programs to perform the functions described herein. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment.

Further Aspects of the Disclosure and its Embodiments

The above described embodiments are directed to charged particle beam microscopes, for example electron beam microscopes. However, alternatively, each of the embodiments can be directed to an ion beam microscope or an inspection system.

Hereinabove, several different embodiments of the disclosure are described. However, the particular configurations of these embodiments can be combined. Further, some parts of these particular configurations of these embodiments can be omitted.