Device using multiple charged particle beams

Abstract

To provide a multi-beam device having high resolution and high throughput for observing or inspecting a sample under various imaging conditions.SOLUTION: The present disclosure proposes an anti-rotation lens and uses an anti-rotation lens as an anti-rotation condenser lens for a multi-beam device having a beamlet preformation mechanism. The anti-rotation condenser lens keeps the angle of rotation of the beamlet unchanged as the current is changed such that the beamlet preformation mechanism can impede the progress of unused electrons as much as possible. Therefore, the multi-beam device can observe a sample with high resolution and high throughput, and is suitable as a yield management tool for inspecting and / or re-examining a defect on a wafer / mask in the semiconductor manufacturing industry.SELECTED DRAWING: Figure 2E

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application No. 62 / 440,493, filed December 30, 2016, which is incorporated herein by reference in its entirety.

[0002]

[0002] The embodiments provided herein disclose charged particle devices using multiple charged particle beams, and more particularly, devices that utilize multiple charged particle beams to observe or inspect a sample. [Background technology]

[0003]

[0003] When manufacturing semiconductor IC chips, pattern defects and / or unwanted particles (residues) inevitably appear on wafers and / or masks during the fabrication process, thereby significantly reducing yields. For example, unwanted particles are very troublesome for patterns with smaller critical feature dimensions that have been introduced to meet increasingly advanced IC chip performance requirements. Accordingly, due to diffraction effects, traditional yield management tools using optical beams are becoming increasingly ineffective, and yield management tools using electron beams are being more frequently adopted because electron beams have shorter wavelengths (compared to photon beams) and can thereby provide superior spatial resolution.

[0004]

[0004] Currently, electron beam-based yield management tools employ the principle of a scanning electron microscope (SEM) using a single electron beam, which can provide higher resolution but cannot provide throughput suitable for mass production. Although throughput can be increased using a higher current of the single electron beam, the good spatial resolution is fundamentally deteriorated by the Coulomb effect, which increases with the beam current.

[0005]

[0005] To overcome the throughput limitations, one solution is to use multiple electron beams, each with a small current, instead of using a single electron beam with a large current. The multiple electron beams form multiple probe spots on the surface of the sample under observation or inspection. The multiple probe spots can simultaneously scan multiple small scanning areas within a large observation area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land, and the secondary electrons form a secondary electron beam.

[0006]

[0006] Secondary electrons include slow secondary electrons (energy ≦50 eV) and backscattered electrons (energy close to the electron landing energy). The secondary electron beams from multiple small scanning regions can be collected simultaneously by multiple electron detectors. As a result, an image of a large observation area including all of the small scanning regions can be obtained much faster than scanning with a single beam.

[0007]

[0007] Multiple electron beams can be obtained from either multiple electron sources or a single electron source. In the former case, the multiple electron beams are usually focused and scan multiple small scanning areas in multiple columns, respectively, and secondary electrons from each scanning area are detected by one electron detector in the corresponding column. Therefore, the apparatus is generally called a multi-column apparatus. On the sample surface, the beam spacing or pitch is on the order of several millimeters to tens of millimeters.

[0008]

[0008] In the latter case, the source conversion unit effectively forms multiple sub-sources from a single electron source. The source conversion unit includes a beamlet limiting (or beamlet forming) mechanism with multiple beam-limiting apertures and an image forming mechanism with multiple electron optical elements. A primary electron beam generated by the single electron source is split into multiple sub-beams or beamlets by the multiple beam-limiting apertures, and the multiple electron optical elements affect the multiple beamlets to respectively form multiple parallel images (virtual or real images) of the single electron source. Each image can be taken as a sub-source emitting a corresponding beamlet. In order to make more beamlets available to achieve higher throughput, the beamlet spacing of the source conversion unit is configured to be as small as possible, typically at the micrometer level.

[0009]

[0009] Within a single column, a primary projection imaging system can be used to project multiple parallel images onto the sample surface and form multiple probe spots on the surface. A common deflection scanning unit can be used to deflect multiple beamlets to scan multiple probe spots across multiple small scanning areas, with each probe spot formed by one beamlet scanning across its own small scanning area. Secondary electron beams generated from the multiple small scanning areas are directed to a secondary projection imaging system by a beam separator and then focused by the secondary projection imaging system to be detected by multiple detection elements of an electron detection device, and each secondary electron beam generated from one small scanning area is detected by one detection element of the electron detection device. The multiple detection elements can be multiple electron detectors arranged side by side or multiple pixels of one electron detector. Therefore, the device is generally referred to as a multi-beam device.

[0010]

[0010] The beamlet limiting mechanism is typically a conductive plate with multiple through holes, which form multiple beam-limiting apertures. In the case of the image-forming mechanism, each electron-optical element can be an electrostatic microlens that focuses one beamlet to form one of multiple parallel real images, or an electrostatic microdeflector that deflects one beamlet to form one of multiple parallel virtual images. The Coulomb effect is weaker for a virtual image than for a real image due to the higher current density associated with the real image.

[0011]

[0011] To reduce the Coulomb effect in the space above the source conversion unit, a beamlet pre-forming mechanism with multiple beamlet-forming apertures can be placed near the single electron source to adjust the primary electron beam as quickly as possible. Most of the electrons in the primary electron beam (but not those used to form the multiple probe spots) cannot pass through the multiple beamlet-forming apertures. The currents of the multiple probe spots can be changed by adjusting the focusing power of a condenser lens between the single electron source and the source conversion unit. Typically, the condenser lens is a magnetic lens, and the multiple beamlets rotate around the optical axis of the condenser lens. The rotation angle changes depending on the focusing power. Variations in the rotation angle can result in a mismatch of the multiple beamlets between the beamlet pre-forming mechanism and the beamlet limiting mechanism, thereby increasing the difference between the currents of the multiple probe spots. The current difference deteriorates the throughput for observing or inspecting the sample. Summary of the Invention

[0012]

[0012] Embodiments of the present disclosure provide a multi-beam device with high resolution and high throughput for observing or inspecting a sample under various imaging conditions (e.g., probe spot current and beamlet landing energy). The embodiments use a beamlet pre-shaping mechanism and an anti-rotation condenser lens or a movable anti-rotation condenser lens to reduce the Coulomb effect, thereby improving the spatial resolution of the image of the sample. The condenser lens can maintain the uniformity of the current of multiple probe spots when changing their current. As a result, the multi-beam device is suitable as a yield management tool for inspecting and / or reviewing defects on wafers / masks in the semiconductor manufacturing industry.

[0013] In some embodiments, an anti-rotation lens having a focusing power for focusing a charged particle beam is provided. The anti-rotation lens includes a first magnetic lens, the first magnetic lens configured to generate a first magnetic field and to be aligned with an optical axis of the anti-rotation lens. The anti-rotation lens also includes a second magnetic lens, the second magnetic lens configured to generate a second magnetic field and to be aligned with the optical axis. The focusing power of the anti-rotation lens is adjustable by varying the first magnetic field and the second magnetic field, the first magnetic field and the second magnetic field having opposite directions on the optical axis.

[0014]

[0014] In some embodiments, an anti-rotation lens having a focusing power for focusing a charged particle beam is provided. The anti-rotation lens includes a magnetic lens, the magnetic lens configured to generate a magnetic field and to be aligned with an optical axis of the anti-rotation lens. The anti-rotation lens also includes an electrostatic lens, the electrostatic lens configured to generate an electrostatic field and to be aligned with the optical axis. The magnetic field and the electrostatic field at least partially overlap, and the focusing power of the anti-rotation lens is adjustable by varying the magnetic field and / or the electrostatic field.

[0015] In some embodiments, a movable anti-rotation lens having a focusing power for focusing a charged particle beam is provided. The movable anti-rotation lens includes a first magnetic lens, the first magnetic lens configured to generate a first magnetic field and to be aligned with an optical axis of the movable anti-rotation lens. The movable anti-rotation lens also includes a second magnetic lens, the second magnetic lens configured to generate a second magnetic field and to be aligned with the optical axis. The movable anti-rotation lens also includes a third magnetic lens, the third magnetic lens configured to generate a third magnetic field and to be aligned with the optical axis. The focusing power and major plane of the movable anti-rotation lens are adjustable by varying the first magnetic field, the second magnetic field, and / or the third magnetic field, wherein two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis.

[0016] In some embodiments, a movable anti-rotation lens having a focusing power for focusing a charged particle beam is disclosed. The movable anti-rotation lens includes an anti-rotation lens configured to be aligned with an optical axis of the movable anti-rotation lens. The movable anti-rotation lens also includes a lens configured to be aligned with the optical axis. The focusing power and a major plane of the movable anti-rotation lens are adjustable by changing the focusing power of the anti-rotation lens and / or the focusing power of the lens, and the major plane is adjustable relative to a source generating the charged particle beam.

[0017] In some embodiments, a movable anti-rotation lens having a focusing power for focusing a charged particle beam is disclosed. The movable anti-rotation lens includes a first anti-rotation lens configured to be aligned with an optical axis of the movable anti-rotation lens. The movable anti-rotation lens also includes a second anti-rotation lens configured to be aligned with the optical axis. The focusing power and principal plane of the movable anti-rotation lens are adjustable by varying the focusing power of the first anti-rotation lens and / or the focusing power of the second anti-rotation lens.

[0018]

[0018] In some embodiments, a multi-beam apparatus for observing a sample is disclosed. The multi-beam apparatus includes an electron source configured to generate a primary electron beam and a focusing lens configured to focus the primary electron beam, the focusing lens being one of an anti-rotation lens or a movable anti-rotation lens. The multi-beam apparatus further includes a source transformation unit configured to form multiple images of the electron source with multiple beamlets of the primary electron beam, an objective lens configured to focus the multiple beamlets on a surface to form multiple probe spots on the surface, and an electron detection device having multiple detection elements configured to detect multiple secondary beams generated by the multiple probe spots from the sample. The multi-beam apparatus may further include a beamlet pre-forming mechanism, the beamlet pre-forming mechanism being between the electron source and the focusing lens and including multiple beamlet-forming apertures. The focusing lens is used to keep the rotation angles of the multiple beamlets unchanged or substantially unchanged when changing the probe currents of the multiple probe spots.

[0019] In some embodiments, a method for configuring an anti-rotation lens for focusing a charged particle beam is provided. The method includes generating a first magnetic field by a first magnetic lens aligned with an optical axis of the anti-rotation lens. The method also includes generating a second magnetic field by a second magnetic lens aligned with the optical axis. The method further includes generating a focusing force of the anti-rotation lens by the first magnetic field and the second magnetic field. The first magnetic field and the second magnetic field have opposite directions on the optical axis.

[0020] In some embodiments, a method is provided for configuring an anti-rotation lens for focusing a charged particle beam. The method includes generating a magnetic field with a magnetic lens and generating an electrostatic field with an electrostatic lens. The method further includes generating a focusing force of the anti-rotation lens with the magnetic field and / or the electrostatic field, wherein the magnetic field and the electrostatic field at least partially overlap.

[0021] In some embodiments, a method for configuring a movable anti-rotation lens for focusing a charged particle beam is provided. The method includes generating a first magnetic field by a first magnetic lens aligned with an optical axis of the movable anti-rotation lens, generating a second magnetic field by a second magnetic lens aligned with the optical axis, and generating a third magnetic field by a third magnetic lens aligned with the optical axis. The method further includes generating a focusing force of the movable anti-rotation lens by the first magnetic field, the second magnetic field, and / or the third magnetic field, wherein two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis.

[0022] In some embodiments, a method for configuring a multi-beam apparatus for observing a sample is provided. The method includes conditioning a primary electron beam from an electron source into multiple beamlets by a beamlet pre-forming mechanism positioned between the electron source and a collector lens, and forming multiple images of the electron source using the multiple beamlets by a source transformation unit. The method further includes forming multiple probe spots on the sample by projecting the multiple images onto the sample, and adjusting the collector lens to keep rotation angles of the multiple beamlets unchanged or substantially unchanged when changing probe currents of the multiple probe spots, wherein the collector lens is one of an anti-rotation lens or a movable anti-rotation lens.

[0023] In some embodiments, a non-transitory computer-readable medium is provided. The non-transitory computer-readable medium stores a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring an anti-rotation lens to focus a charged particle beam. The method includes directing a first magnetic lens to generate a first magnetic field, the first magnetic lens being aligned with an optical axis of the anti-rotation lens, and directing a second magnetic lens to generate a second magnetic field, the second magnetic lens being aligned with the optical axis. The first magnetic field and the second magnetic field generate focusing forces of the anti-rotation lens and have opposite directions on the optical axis.

[0024] In some embodiments, a non-transitory computer-readable medium is provided. The non-transitory computer-readable medium stores a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring an anti-rotation lens to focus a charged particle beam. The method includes instructing a magnetic lens to generate a magnetic field and instructing an electrostatic lens to generate an electrostatic field. The magnetic field and / or the electrostatic field generate a focusing force of the anti-rotation lens. Moreover, the magnetic field and the electrostatic field at least partially overlap.

[0025] In some embodiments, a non-transitory computer-readable medium is provided. The non-transitory computer-readable medium stores a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring a movable anti-rotation lens to focus a charged particle beam. The method includes: directing a first magnetic lens to generate a first magnetic field, where the first magnetic lens is aligned with an optical axis of the movable anti-rotation lens; directing a second magnetic lens to generate a second magnetic field, where the second magnetic lens is aligned with the optical axis; and directing a third magnetic lens to generate a third magnetic field, where the third magnetic lens is aligned with the optical axis. The first magnetic field, the second magnetic field, and / or the third magnetic field generate a focusing force of the movable anti-rotation lens. Moreover, two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis.

[0026]

[0026] Other advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which are set forth, by way of illustration and example, certain embodiments of the invention. [Brief explanation of the drawings]

[0027] [Figure 1A]

[0027] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device. [Figure 1B]

[0027] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device. [Figure 2A]

[0028] 1C is a cross-sectional view in the XY plane of the beamlet pre-forming mechanism of FIGS. 1A and 1B, showing exemplary spots of the primary electron beam and three beamlets. [Figure 2B]

[0029] 1A and 1B, showing exemplary spots of three beamlets and three beam-limiting apertures. FIG. [Figure 2C]

[0029] FIG. 1C is a cross-sectional view in the XY plane of the beamlet limiting mechanism of FIGS. 1A and 1B, showing exemplary spots of three beamlets and three beam-limiting apertures. [Figure 2D]

[0029] FIG. 1C is a cross-sectional view in the XY plane of the beamlet limiting mechanism of FIGS. 1A and 1B, showing exemplary spots of three beamlets and three beam-limiting apertures. [Figure 2E]

[0029] FIG. 1C is a cross-sectional view in the XY plane of the beamlet limiting mechanism of FIGS. 1A and 1B, showing exemplary spots of three beamlets and three beam-limiting apertures. [Figure 3A]

[0030] 1A-1C are schematic diagrams illustrating exemplary configurations of anti-rotation lenses consistent with embodiments of the present disclosure. [Figure 3B]

[0030] FIG. 1 is a schematic diagram illustrating an exemplary configuration of an anti-rotation lens consistent with an embodiment of the present disclosure. [Figure 3C]

[0031] FIG. 3B is a schematic diagram illustrating the example magnetic field distribution of FIG. 3A, consistent with an embodiment of the present disclosure. [Figure 3D] FIG. 3B is a schematic diagram illustrating the example magnetic field distribution of FIG. 3A, consistent with an embodiment of the present disclosure. [Figure 4A]

[0032] 1A-1C are schematic diagrams illustrating exemplary configurations of anti-rotation lenses consistent with embodiments of the present disclosure. [Figure 4B]

[0032] FIG. 1 is a schematic diagram illustrating an exemplary configuration of an anti-rotation lens consistent with an embodiment of the present disclosure. [Figure 4C]

[0032] FIG. 1 is a schematic diagram illustrating an exemplary configuration of an anti-rotation lens consistent with an embodiment of the present disclosure. [Figure 5A]

[0033] 1A and 1B are schematic diagrams illustrating exemplary configurations of movable anti-rotation lenses consistent with embodiments of the present disclosure. [Figure 5B]

[0034] FIG. 5B is a schematic diagram illustrating the example magnetic field distribution of FIG. 5A, consistent with an embodiment of the present disclosure. [Figure 5C]FIG. 5B is a schematic diagram illustrating the example magnetic field distribution of FIG. 5A, consistent with an embodiment of the present disclosure. [Figure 5D] FIG. 5B is a schematic diagram illustrating the example magnetic field distribution of FIG. 5A, consistent with an embodiment of the present disclosure. [Figure 6A]

[0035] 1A and 1B are schematic diagrams illustrating exemplary configurations of movable anti-rotation lenses consistent with embodiments of the present disclosure. [Figure 6B]

[0035] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a movable anti-rotation lens consistent with an embodiment of the present disclosure. [Figure 7A]

[0036] 1A and 1B are schematic diagrams illustrating exemplary configurations of movable anti-rotation lenses consistent with embodiments of the present disclosure. [Figure 7B]

[0036] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a movable anti-rotation lens consistent with an embodiment of the present disclosure. [Figure 8A]

[0037] 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 8B]

[0038] 8B illustrates an exemplary configuration of the beamlet pre-forming mechanism of FIG. 8A, consistent with embodiments of the present disclosure. [Figure 8C]

[0038] Figure 8B illustrates an exemplary configuration of the beamlet pre-forming mechanism of Figure 8A, consistent with embodiments of the present disclosure. [Figure 9A]

[0039] 1 is a schematic diagram illustrating an example configuration of a multi-beam device and beam paths consistent with an embodiment of the present disclosure. FIG. [Figure 9B]

[0040] 9C is a schematic diagram illustrating an example configuration of the beamlet pre-forming mechanism of FIGS. 9A and 9C, consistent with an embodiment of the present disclosure. FIG. [Figure 9C]

[0039] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device and beam paths consistent with an embodiment of the present disclosure. [Figure 10]

[0041] 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 11A]

[0042] 1 is a schematic diagram illustrating an example configuration of a multi-beam device and beam paths consistent with an embodiment of the present disclosure. FIG. [Figure 11B]

[0043] 11A and 11C are schematic diagrams illustrating exemplary configurations of the beamlet pre-forming mechanism of FIGS. 11A and 11C, consistent with embodiments of the present disclosure. [Figure 11C]

[0042] A schematic diagram showing an exemplary configuration of a multi-beam device and beam paths consistent with an embodiment of the present disclosure. [Figure 12A]

[0044] 1 is a schematic diagram illustrating an example configuration of a beamlet pre-forming mechanism, consistent with embodiments of the present disclosure. FIG. [Figure 12B]

[0044] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a beamlet pre-forming mechanism consistent with an embodiment of the present disclosure. [Figure 13]

[0045] 12B is a schematic diagram illustrating an example configuration of a multi-beam device with a beamlet pre-forming mechanism of FIG. 12A, consistent with an embodiment of the present disclosure. [Figure 14]

[0046] 12B is a schematic diagram illustrating an example configuration of a multi-beam device with a beamlet pre-forming mechanism of FIG. 12A, consistent with an embodiment of the present disclosure. [Figure 15A]

[0047] 1 is a schematic diagram illustrating an exemplary configuration of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 15B]

[0048] FIG. 15B is a schematic diagram illustrating the scan path of the beamlets on the sample surface of FIG. 15A, consistent with an embodiment of the present disclosure. [Figure 15C]

[0049] FIG. 15B is a schematic diagram illustrating the scanning path of a secondary beam on a detection element of the electronic detection device of FIG. 15A, consistent with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0028]

[0050] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings, in which like numbers in different drawings represent the same or similar elements unless otherwise stated. The implementations described in the following description of exemplary embodiments do not represent all implementations consistent with the present invention. Instead, the implementations are merely examples of apparatus and methods consistent with aspects related to the present invention, as set forth in the appended claims.

[0029]

[0051] In the drawings, the relative dimensions of the components may be exaggerated for clarity. Within the following description of the drawings, the same or similar reference numbers refer to the same or similar components or entities, and only the differences with respect to the individual embodiments are described.

[0030]

[0052] Without limiting the scope of protection, the descriptions and drawings of the embodiments are all considered to be electron beams by way of example, but the embodiments are not used to limit the disclosed embodiments to any particular charged particle.

[0031]

[0053] FIELD OF THE INVENTION

[0002] Embodiments of the present application relate to multi-beam or charged particle devices using multiple charged particle beams. More specifically, embodiments of the present application relate to devices employing multiple charged particle beams to simultaneously acquire images of multiple scanned regions of an observation area on a sample surface. The devices can be used in semiconductor manufacturing to inspect and / or review defects on wafers / masks with high resolution and high throughput.

[0032]

[0054] Furthermore, embodiments of the present application relate to a beamlet pre-forming mechanism and an anti-rotation condenser lens or a movable anti-rotation condenser lens for reducing the Coulomb effect in a multi-beam device and thus improving the spatial resolution of images of multiple small scan areas of a sample under observation or inspection. In a multi-beam device, a beamlet pre-forming mechanism having multiple beamlet-forming apertures conditions a primary electron beam from a single electron source into multiple beamlets. The anti-rotation condenser lens or the movable anti-rotation condenser lens focuses the multiple beamlets to be incident on a source transformation unit, which forms multiple parallel images of the single electron source with the multiple beamlets. The parallel images are projected onto the sample surface (via an objective lens) to form multiple probe spots on the surface. The multiple beamlets can be deflected by at least one of a deflection scanning unit and a source transformation unit to scan the multiple probe spots over the small scan area, respectively. The current of the probe spot can be limited by the source transformation unit and varied by adjusting the anti-rotation focusing lens, or limited by the beamlet pre-forming mechanism and varied by changing the size of the beamlet forming aperture. The source transformation unit can reduce the size of the probe spot and the size difference between the probe spots by compensating for its off-axis aberrations.

[0033]

[0055] Furthermore, the disclosed embodiments provide a beamlet pre-forming mechanism, an anti-rotation focusing lens, a movable anti-rotation focusing lens, and corresponding configurations of a multi-beam device. For illustrative purposes, three beamlets are shown in most embodiments, but the actual number of beamlets can be any. Deflection scanning units, beam separators, secondary projection imaging systems, electronic detection devices, and any combination thereof can be incorporated into the multi-beam device and are not shown or occasionally mentioned in the description of the embodiments.

[0034]

[0056] According to an embodiment of the present disclosure, the X, Y, and Z axes are Cartesian coordinates: the main optical axis of the multi-beam device is on the Z axis, and the primary electron beam from the single electron source travels along the Z axis.

[0035]

[0057] When an electron beam passes through a magnetic lens whose optical axis is on the Z axis, the focusing force 1 / f of the magnetic lens and the rotation angle θ of the electron beam around the optical axis are determined by its magnetic field and can be calculated by equations (1) and (2).

number

[0036]

[0058] 1A, which is a schematic diagram illustrating an exemplary configuration of a conventional multi-beam apparatus 100A. The multi-beam apparatus 100A includes an electron source 101, a beamlet pre-forming mechanism 172, a condenser lens 110, a source conversion unit 120, and an objective lens 131. Beamlet-forming apertures 172_1, 172_2, and 172_3 of the beamlet pre-forming mechanism 172, located close to the electron source 101 and far above the source conversion unit 120, condition a primary electron beam 102 generated by the electron source 101 into three beamlets 102_1, 102_2, and 102_3 (102_2 and 102_3 are off-axis beamlets that are not on the main optical axis 100_1 of the apparatus 100A). The condenser lens 110 focuses the three beamlets 102_1 to 102_3 to enter the source conversion unit 120. The source conversion unit 120 includes one beamlet pre-bending mechanism 123 having three pre-bending micro-deflectors 123_1, 123_2, and 123_3, one beamlet limiting mechanism 121 having three beam-limiting apertures 121_1 to 121_3, and one image forming mechanism 122 having three image forming micro-deflectors 122-1_1, 122-1_2, and 122-1_3. The three pre-bending micro-deflectors 123_1 to 123_3 deflect the three beamlets 102_1 to 102_3 so that they are perpendicularly incident on the three beam-limiting apertures 121_1 to 121_3, respectively. The beam-limiting apertures 121_1, 121_2, and 121_3 prevent the progression of residual peripheral electrons in the three beamlets 102_1 to 102_3, thus limiting their current. The three image-forming microdeflectors 122-1_1 to 122-1_3 deflect the three beamlets 102_1 to 102_3 toward the main optical axis 100_1, forming three virtual images of the electron source 101. The objective lens 131 focuses the three beamlets 102_1 to 102_3 onto the surface 7 of the sample 8 (i.e., projects three virtual images onto the surface 7). The three images formed on the surface 7 by the three beamlets 102_1 to 102_3 form three probe spots 102_1S, 102_2S, and 102_3S on the surface.The deflection angles of the beamlets 102_1 to 102_3 by the image-forming micro-deflectors 122-1_1 to 122-1_3 are adjusted to reduce off-axis aberrations of the three probe spots 102_1S to 102_3S by the objective lens 131, and as a result, the three deflected beamlets pass through or approach the pre-focus point of the objective lens 131.

[0037]

[0059] The current can be changed by adjusting the focusing power of the focusing lens 110. However, the positions of the beamlets 102_1 to 102_3 change relative to the beam-limiting apertures 121_1 to 121_3 as the current changes. Therefore, misalignment between one beamlet coming from the beamlet-forming aperture and its corresponding beam-limiting aperture may occur. The focusing lens 110 can be configured as an electrostatic, magnetic, or electromagnetic composite lens. A magnetic focusing lens has smaller aberrations compared to an electrostatic lens. In the case of an electrostatic focusing lens, the misalignment occurs only along the radial direction, which is perpendicular to the main optical axis 100_1, as shown in FIG. 2C. In the case of a magnetic or electromagnetic composite lens, the misalignment occurs along both the radial and rotational directions as the rotation angle of the beamlets 102_1 to 102_3 changes, as shown in FIG. 2E.

[0038]

[0060] In some embodiments, the condenser lens 110 is configured to be magnetic, which may cause the off-axis beamlets 102_2, 102_3 to pass through the beamlet pre-forming mechanism 172 and land on the beamlet-limiting mechanism 121 at a rotation angle. The rotation angle varies depending on the focusing power of the condenser lens 110. Completely filling the beam-limiting apertures 121_2, 121_3 with the beamlets 102_2, 102_3 can be achieved by increasing the size of the beamlet-forming apertures 172_2, 172_3. Increasing the size of the beamlet-forming apertures may introduce more unused electrons, thus increasing the Coulomb effect in the space between the beamlet pre-forming mechanism 172 and the source conversion unit 120. The spatial resolution of the image of a small scanning area may deteriorate. Complete filling of the beam-limiting apertures 121_2, 121_3 with the beamlets 102_2, 102_3 can also be achieved by keeping the rotation angle unchanged or substantially unchanged when changing the focusing power of the collecting lens 110. This solution uses an anti-rotation lens as the collecting lens 110.

[0039]

[0061] 1B, which is a schematic diagram showing another exemplary configuration of a multi-beam apparatus 200A. The multi-beam apparatus 200A includes an electron source 101, a beamlet pre-forming mechanism 172, a movable condenser lens 210, a source conversion unit 220, and an objective lens 131. A primary electron beam 102 generated by the electron source 101 is adjusted into three beamlets 102_1, 102_2, and 102_3 by three beamlet-forming apertures 172_1, 172_2, and 172_3 of the beamlet pre-forming mechanism 172. The movable condenser lens 210 focuses the three beamlets 102_1 to 102_3 so that they are perpendicularly incident on the source conversion unit 220. The source conversion unit 220 includes one beamlet limiting mechanism 121 having three beam-limiting apertures 121_1 to 121_3 and one image-forming mechanism 122-1 having three image-forming micro-deflectors 122-1_1, 122-1_2, and 122-1_3. The beam-limiting apertures 121_1, 121_2, and 121_3 prevent the remaining peripheral electrons of the three beamlets 102_1 to 102_3 from traveling, thus limiting their current. The three image-forming micro-deflectors 122-1_1 to 122-1_3 deflect the three beamlets 102_1 to 102_3 toward the main optical axis 200_1, forming three virtual images of the electron source 101. The objective lens 131 focuses the three beamlets 102_1 to 102_3 onto the surface 7 of the sample 8 (i.e., projects three virtual images onto the surface 7). The three images formed on the surface 7 by the three beamlets 102_1 to 102_3 form three probe spots 102_1S, 102_2S, and 102_3S on the surface. The deflection angles of the beamlets 102_1 to 102_3 by the imaging micro-deflectors 122-1_1 to 122-1_3 are adjusted to reduce off-axis aberrations of the three probe spots 102_1S to 102_3S formed by the objective lens 131, so that the three deflected beamlets pass through or approach the pre-focal point of the objective lens 131.

[0040]

[0062] The movable focusing lens 210 is configured such that the position of its first principal plane 210_2 is movable. The current of the beamlets 102_1-102_3 can be changed by adjusting both the focusing power of the movable focusing lens 210 and the position of the first principal plane 210_2 while holding the beamlets 102_1-102_3 perpendicularly incident on the source transformation unit 120. However, the positions of the beamlets 102_1-102_3 change with respect to the beam-limiting apertures 121_1-121_3 as the current changes. Misalignment between one beamlet coming from the beamlet-forming aperture and its corresponding beam-limiting aperture may occur. The movable focusing lens 210 may be configured to be an electrostatic, magnetic, or electromagnetic composite lens. As mentioned above, the misalignment occurs only along the radial direction (perpendicular to the main optical axis 200_1) in the case of an electrostatic focusing lens, or along both the radial and rotational directions in the case of a magnetic or electromagnetic composite focusing lens.

[0041]

[0063] In some embodiments, the movable condenser lens 210 is configured to be magnetic, which may cause the off-axis beamlets 102_2, 102_3 to pass through the beamlet pre-forming mechanism 172 and land on the beamlet-limiting mechanism 121 at a rotation angle. The rotation angle varies depending on the focusing power of the movable condenser lens 210 and the position of the first principal plane. Completely filling the beam-limiting apertures 121_2, 121_3 with the beamlets 102_2, 102_3 can be achieved by increasing the size of the beamlet-forming apertures 172_2, 172_3. Increasing the size of the beamlet-forming apertures may introduce more unused electrons, thereby increasing the Coulomb effect in the space between the beamlet pre-forming mechanism 172 and the source conversion unit 120. Therefore, the spatial resolution of the image of a small scanning area may deteriorate. Complete filling of the beam-limiting apertures 121_2, 121_3 with the beamlets 102_2, 102_3 can also be achieved by keeping the rotation angle unchanged while changing the focusing power and the position of the first principal plane of the movable focusing lens 210. This solution uses an anti-rotation lens with a movable first principal plane (i.e., a movable anti-rotation lens (MARL)) as the movable focusing lens 210.

[0042]

[0064] Reference is now made to Figures 2A-2E. Figure 2A is a cross-sectional view in the XY plane of the beamlet pre-forming mechanism 172 of Figures 1A and 1B, showing the primary electron beam and exemplary spots of three beamlets. Figures 2B-2E are cross-sectional views in the XY plane of the beamlet limiting mechanism 121 of Figures 1A and 1B, showing exemplary spots of three beamlets and three beam-limiting apertures, respectively.

[0043]

[0065] In Fig. 2A, the primary electron beam 102 is incident on the beamlet pre-forming mechanism 172, where the beamlet-forming apertures 172_1 to 172_3 are arranged along the X-axis. If the condenser lens 110 in Fig. 1A or the movable condenser lens 210 in Fig. 1B are electrostatic, the beamlets 102_1 to 102_3 may be incident on the beamlet limiting mechanism 121 along the X-axis, as shown in Figs. 2B and 2C. The three beamlets 102_1 to 102_3 are more tightly focused in Fig. 2C than in Fig. 2B. Accordingly, the currents of the three beamlets 102_1 to 102_3 are greater in Fig. 2C than in Fig. 2B. If the focusing lens 110 in Figure 1A or the movable focusing lens 210 in Figure 1B is a magnetic or electromagnetic composite, the beamlets 102_1-102_3 are incident on the beamlet limiting mechanism 121 at a rotation angle around the optical axis of the corresponding focusing lens, as shown in Figures 2D and 2E. The beamlets 102_1-102_3 are more strongly focused in Figure 2E than in Figure 2D. Accordingly, the beamlets 102_1-102_3 are incident on the beamlet limiting mechanism 121 at a larger current and a larger rotation angle in Figure 2E than in Figure 2D. In FIG. 2E, the two edge beamlets 102_2, 102_3 are rotated partially out of the beam-limiting apertures 121_2, 121_3, and if the beam-limiting apertures 121_1 to 121_3 have the same radial size, their currents after passing through the beam-limiting apertures 121_2, 121_3 may be different from the current of the central beamlet 102_1.

[0044]

[0066] An anti-rotation lens has a focusing power that can be changed without affecting the rotation angle of the electron beam passing through it. If the focusing lens 110 in FIG. 1A is an anti-rotation lens, the discrepancy in FIG. 2E can be eliminated. The anti-rotation lens can be formed by two magnetic lenses or one magnetic lens and one electrostatic lens. By appropriately adjusting the excitation of the lenses in the anti-rotation lens relative to the electron beam, the focusing power can be changed without affecting the rotation angle of the electron beam.

[0045]

[0067] Reference is now made to Figures 3A and 3B, which are each schematic diagrams illustrating an exemplary configuration of an anti-rotation lens including two magnetic lenses consistent with embodiments of the present disclosure, while reference is also made to Figures 3C and 3D, which are schematic diagrams illustrating exemplary magnetic field distributions of Figures 3A and 3B, consistent with embodiments of the present disclosure.

[0046]

[0068] In FIG. 3A, the two single magnetic lenses ARL-1-1 and ARL-1-2 of the anti-rotation lens ARL-1 are configured to be aligned with its optical axis ARL-1_1 (on the Z axis). The magnetic lenses ARL-1-1 and ARL-1-2 are excited to generate a magnetic field, which is a magnetic field B on the optical axis ARL-1_1. ARL-1-1 (z), B ARL-1-2 The distribution of (z) is opposite in polarity as shown in Figure 3C or Figure 3D. ARL-1-1 (z), B ARL-1-2 (z) can be adjusted based on an appropriate ratio, and the focusing power can be adjusted accordingly without affecting the overall rotation angle of the electron beam passing through the anti-rotation lens ARL-1.

[0047]

[0069] It is understood that in some embodiments, two single magnetic lenses can be connected to partially share a common magnetic circuit. For example, as shown in FIG. 3B, two magnetic lenses ARL-2-1, ARL-2-2 of anti-rotation lens ARL-2 are configured to partially share a common magnetic circuit between them.

[0048]

[0070] 3A also shows how anti-rotation lens ARL-1 functions when used as a focusing lens in a multi-beam device, such as focusing lens 110 shown in FIG. 1A. For example, as shown in FIG. 3C, magnetic field B ARL-1-1 (z), B ARL-1-2 When (z) is set to have the same distribution but opposite polarity, the primary electron beam 102 is focused as shown by the solid line in FIG. 3A. ARL-1-1(z), B ARL-1-2 If the primary electron beam 102 is not subjected to a total on-axis magnetic field B(z), which is the sum of all on-axis magnetic fields B(z), the overall rotation angle of the primary electron beam 102 after exiting the anti-rotation lens ARL-1 is zero. As shown in FIG. 3D, B ARL-1-1 (z), B ARL-1-2 When (z) increases equally, the focusing power of the primary electron beam 102 increases (e.g., as shown by the dashed line in FIG. 3A), and the rotation angle of the primary electron beam 102 after exiting the anti-rotation lens ARL-1 remains zero. When a single electron source 101 is subjected to a full on-axis magnetic field B(z), the rotation angle with respect to FIG. 3C is non-zero. B ARL-1-1 (z), B ARL-1-2 (z) can be increased in an appropriate ratio to increase the focusing power while keeping the rotation angle unchanged.

[0049]

[0071] Reference is now made to Figures 4A, 4B, and 4C, each of which is a schematic diagram illustrating an exemplary configuration of an anti-rotation lens including an electrostatic lens and a magnetic lens consistent with an embodiment of the present disclosure. In Figure 4A, the anti-rotation lens ARL-3 includes an electrostatic lens ARL-3-1 formed by three electrodes ARL-3-1_e1, ARL-3-1_e2, and ARL-3-1_e3, and a magnetic lens ARL-3-2. The electrostatic field of the electrostatic lens ARL-3-1 and the magnetic field of the magnetic lens ARL-3-2 are configured to partially overlap each other. Accordingly, the energy of an electron beam passing through the magnetic field is changed by the electrostatic field, and its rotation angle depends on both the magnetic field and the electrostatic field. Therefore, by changing the magnetic field and the electrostatic field in an appropriate ratio, the focusing power of the anti-rotation lens ARL-3 can be changed without affecting the rotation angle of the electron beam after exiting the anti-rotation lens ARL-3.

[0050]

[0072] 4B, the anti-rotation lens ARL-4 includes an electrostatic lens ARL-4-1 formed by three electrodes ARL-4-1_e1, ARL-4-1_e2, and ARL-4-1_e3, and a magnetic lens ARL-4-2. The electrostatic field of the electrostatic lens ARL-4-1 and the magnetic field of the magnetic lens ARL-4-2 are configured to completely overlap with each other, thereby more efficiently realizing keeping the rotation angle unchanged.

[0051]

[0073] In Figure 4C, the anti-rotation lens ARL-5 includes an electrostatic lens ARL-5-1 and a magnetic lens ARL-5-2. The electrostatic lens ARL-5-1 is formed by two end electrodes ARL-5-1_e1 and ARL-5-1_e3 and a central electrode ARL-5-1_e2 located between them. The two end electrodes ARL-5-1_e1 and ARL-5-1_e3 also serve as two pole pieces for the magnetic lens ARL-5-2. This makes the configuration more compact and simple.

[0052]

[0074] The movable anti-rotation lens has a movable first principal plane. The position of the first principal plane and the focusing power of the movable anti-rotation lens can be changed relative to the electron beam without affecting the rotation angle of the electron beam. If the movable focusing lens 210 of FIG. 1B is a movable anti-rotation lens, the discrepancy in FIG. 2E can be eliminated. The movable anti-rotation lens can be formed by three magnetic lenses, one anti-rotation lens and one conventional lens (electrostatic or magnetic), or two anti-rotation lenses. By appropriately adjusting the excitation of the lenses in the movable anti-rotation lens, the first principal plane and the focusing power can be changed simultaneously relative to the electron beam without affecting the rotation angle of the electron beam.

[0053]

[0075] Reference is now made to FIG. 5A, which is a schematic diagram illustrating an exemplary configuration of a movable anti-rotation lens including three magnetic lenses, consistent with an embodiment of the present disclosure. Meanwhile, reference is also made to FIGS. 5B, 5C, and 5D, which are schematic diagrams illustrating exemplary magnetic field distributions of FIG. 5A, consistent with an embodiment of the present disclosure. In FIG. 5A, the three single magnetic lenses MARL-1-1, MARL-1-2, and MARL-1-3 of the movable anti-rotation lens MARL-1 are aligned with its optical axis MARL-1_1 (on the Z axis). Two or all of the single magnetic lenses MARL-1-1 through MARL-1-3 can be connected and configured to partially share a common magnetic circuit. The magnetic lenses MARL-1-1 through MARL-1-3 are excited to generate a magnetic field, and the on-axis distribution of the magnetic field (the field on the optical axis MARL-1_1) is expressed as B MARL-1-1 (z), B MARL-1-2 (z), B MARL-1-3 (z). Axial magnetic field B MARL-1-1 (z), B MARL-1-2 (z), B MARL-1-3 Two of the (z) have opposite polarities as shown in Figures 5B to 5D. The excitation of the magnetic lenses MARL-1-1 to MARL-1-3 is performed by the axial magnetic field B MARL-1-1 (z), B MARL-1-2 (z), B MARL-1-3 The total axial magnetic field distribution, which is the sum of (z), is set to vary, thereby moving the first principal plane and adjusting the focusing power of the movable anti-rotation lens MARL-1. Within the total axial magnetic field distribution, its polarity alternates between being along the Z axis and being opposite to the Z axis in order to keep the overall rotation angle unchanged.

[0054]

[0076] 5A also illustrates how the movable anti-rotation lens functions when used as a movable condenser lens in a multi-beam device, such as movable condenser lens 210 of FIG. 1B. For example, as shown in FIG. 5B, magnetic field B MARL-1-1 (z), B MARL-1-2 (z) have the same distribution but opposite polarity, and the magnetic field B MARL-1-3When the excitation of the on-axis magnetic lenses MARL-1-1 to MARL-1-3 is set so that (z) is equal to zero, the first principal plane is in plane P1-3 in FIG. 5A, which is close to the single electron source 101. The primary electron beam 102 from the single electron source 101 becomes parallel in plane P1-3 and has a beam width 102W_P1-3 after exiting the movable condenser lens MARL-1. If the single electron source 101 is not exposed to a full on-axis magnetic field, the rotation angle of the primary electron beam 102 after exiting the movable condenser lens MARL-1 is zero. As shown in FIG. 5C, the on-axis magnetic field B MARL-1-1 (z), B MARL-1-2 (z), B MARL-1-3 By changing the excitation of the magnetic lens to adjust (z), the first principal plane moves away from the single electron source 101 to plane P1-1, as shown in FIG. 5A. The primary electron beam 102 becomes parallel at plane P1-1 and has an increased beam width 102W_P1-1, with the same rotation angle as when the beam width was 102W_P1-3. As shown in FIG. 5D, the on-axis magnetic field B MARL-1-1 (z), B MARL-1-2 (z), B MARL-1-3 By further changing the excitation of the magnetic lens to adjust (z), the first principal plane moves further away from the single electron source 101 to plane P1-2, as shown in FIG. 5A. The primary electron beam 102 becomes parallel at plane P1-2 and has a further increased beam width 102W_P1-2, with the same rotation angle as when the beam width was 102W_P1-3 or 102W_P1-1. The first principal plane can be configured to move between plane P1-3 and plane P1-2, so that the primary electron beam 102 can exit parallel with a changed beam width and an unchanged rotation angle.

[0055]

[0077] Reference is now made to FIG. 6A, which is a schematic diagram illustrating an exemplary configuration of a movable anti-rotation lens having one anti-rotation lens and a conventional electrostatic lens consistent with an embodiment of the present disclosure. The anti-rotation lens MARL-2-1 and electrostatic lens MARL-2-2 of the movable anti-rotation lens MARL-2 are aligned with its optical axis MARL-2_1. Anti-rotation lens MARL-2-1 includes two magnetic lenses MARL-2-1-1 and MARL-2-1-2, similar to the anti-rotation lens ARL-1 of FIG. 3A. The electrostatic lens MARL-2-2 includes three electrodes MARL-2-2_e1, MARL-2-2_e2, and MARL-2-2_e3. The first principal plane of the movable anti-rotation lens MARL-2 can be configured to be located at plane P2-3 between the two single magnetic lenses MARL-2-1-1 and MARL-2-1-2 in response to the electrostatic lens MARL-2-2 being off and the anti-rotation lens MARL-2-1 being on. The first principal plane can also be configured to be located at plane P2-2 near the central electrode MARL-2-2_e2 between the two end electrodes MARL-2-2_e3 and MARL-2-2_e1 in response to the anti-rotation lens MARL-2-1 being off and the electrostatic lens MARL-2-2 being on. By adjusting the focusing power ratio between the anti-rotation lens MARL-2-1 and the electrostatic lens MARL-2-2, the first principal plane can be configured to move between planes P2-3 and P2-2. The anti-rotation lens MARL-2-1 can keep the rotation angle unchanged and equal to a specific value while moving the first principal plane. The electrostatic lens MARL-2-2 does not have a rotation function. Therefore, if the specific value is not zero, the electrostatic lens MARL-2-2 must work together with the anti-rotation lens MARL-2-1. As a result, the range of movement of the first principal plane is reduced.

[0056]

[0078] Reference is now made to Figure 6B, which is a schematic diagram illustrating an exemplary configuration of a movable anti-rotation lens having one anti-rotation lens and a conventional magnetic lens consistent with an embodiment of the present disclosure. The anti-rotation lens MARL-3-1 and magnetic lens MARL-3-2 of the movable anti-rotation lens MARL-3 are aligned with its optical axis MARL-3_1. Anti-rotation lens MARL-3-1 includes one electrostatic lens MARL-3-1-1 and one magnetic lens MARL-3-1-2, similar to anti-rotation lens ARL-5 of Figure 4C.

[0057]

[0079] The first principal plane of movable anti-rotation lens MARL-3 can be configured to be located in plane P3-3 intersecting electrode MARL-3-1_e2 of anti-rotation lens MARL-3-1 in response to magnetic lens MARL-3-2 being off and anti-rotation lens MARL-3-1 being on. The first principal plane can also be configured to be located in plane P3-2 between the two pole pieces of magnetic lens MARL-3-2 in response to anti-rotation lens MARL-3-1 being off and magnetic lens MARL-3-2 being on. The first principal plane can be configured to move between planes P3-3 and P3-2 by adjusting the focusing power ratio between anti-rotation lens MARL-3-1 and magnetic lens MARL-3-2. The rotation angle due to anti-rotation lens MARL-3-1 can be adjusted relative to the rotation angle due to magnetic lens MARL-3-2 in order to keep the overall rotation angle due to movable anti-rotation lens MARL-3 unchanged while moving the first principal plane.

[0058]

[0080] Reference is now made to Figures 7A and 7B, which are schematic diagrams illustrating an exemplary configuration of a movable anti-rotation lens having two anti-rotation lenses, each consistent with an embodiment of the present disclosure. In Figure 7A, the two anti-rotation lenses MARL-4-1 and MARL-4-2 of movable anti-rotation lens MARL-4 are aligned with its optical axis MARL-4_1. Anti-rotation lens MARL-4-1 includes magnetic lenses MARL-4-1-1 and MARL-4-1-2. Anti-rotation lens MARL-4-2 includes magnetic lenses MARL-4-2-1 and MARL-4-2-2. Both anti-rotation lenses MARL-4-1 and MARL-4-2 are similar to anti-rotation lens ARL-1 of Figure 3A. The first principal plane of movable anti-rotation lens MARL-4 can be configured to be located at plane P4-3 between magnetic lenses MARL-4-1-1 and MARL-4-1-2 in response to anti-rotation lens MARL-4-1 being on and anti-rotation lens MARL-4-2 being off. The first principal plane can also be configured to be located at plane P4-2 between magnetic lenses MARL-4-2-1 and MARL-4-2-2 in response to anti-rotation lens MARL-4-1 being off and anti-rotation lens MARL-4-2 being on. By adjusting the focusing power ratio of anti-rotation lenses MARL-4-1 and MARL-4-2, the first principal plane can be configured to move between planes P4-3 and P4-2. The rotation angle through each of the anti-rotation lenses MARL-4-1, MARL-4-2 can be adjusted to zero to eliminate the overall rotation angle, or can be set to keep the overall rotation angle unchanged while moving the first principal plane.

[0059]

[0081] In Figure 7B, anti-rotation lenses MARL-5-1 and MARL-5-2 of movable anti-rotation lens MARL-5 are aligned with its optical axis MARL-5_1. Anti-rotation lens MARL-5-1 includes magnetic lenses MARL-1-1 and MARL-5-1-2, similar to anti-rotation lens ARL-1 of Figure 3A. Anti-rotation lens MARL-5-2 includes electrostatic lens MARL-5-2-1 and magnetic lens MARL-5-2-2, similar to anti-rotation lens ARL-5 of Figure 4C. The first principal plane of movable anti-rotation lens MARL-5 can be positioned at plane P5-3 within anti-rotation lens MARL-5-1 in response to anti-rotation lens MARL-5-1 being on and anti-rotation lens MARL-5-2 being off. The first principal plane can also be located in plane P5-2, which intersects with electrode MARL-5-2-1_e2 of anti-rotation lens MARL-5-2, in response to anti-rotation lens MARL-5-1 being off and anti-rotation lens MARL-5-2 being on. By adjusting the focusing power ratio between anti-rotation lenses MARL-5-1 and MARL-5-2, the first principal plane can be configured to move between planes P5-3 and P5-2. The rotation angle due to anti-rotation lens MARL-5-2 is typically not equal to zero (it is equal to zero only when magnetic lens MARL-5-2-2 is off). The rotation angle due to anti-rotation lens MARL-5-1 can be adjusted accordingly in response to anti-rotation lens MARL-5-2 to keep the overall rotation angle unchanged while moving the first principal plane. In other words, the rotation angle due to the anti-rotation lens MARL-5-1 cannot be zero when the anti-rotation lens MARL-5-2 is off, and can be zero when the anti-rotation lens MARL-5-2 is on. Accordingly, the plane P5-3 is close to one of the magnetic lenses MARL-5-1-1 or MARL-5-1-2. It is preferable to place the plane P5-3 close to the magnetic lens MARL-5-1-1 because this arrangement increases the available range between the planes P5-3 and P5-2.

[0060]

[0082] In a multi-beam device using a focusing lens for varying the current of multiple beamlets and a beamlet pre-forming mechanism for reducing the Coulomb effect, such as that shown in FIG. 1A, the Coulomb effect can be further reduced if the focusing lens is configured to be an anti-rotation focusing lens with the anti-rotation lens described above. The anti-rotation focusing lens can keep the rotation angles of the multiple beamlets unchanged or substantially unchanged when varying their currents, thereby eliminating the mismatch between the multiple beamlets and the corresponding beam-limiting apertures. Accordingly, the size of the beamlet-forming aperture of the beamlet pre-forming mechanism does not need to be enlarged to cover the mismatch, thereby preventing more unused electrons from passing through.

[0061]

[0083] Reference is now made to Figure 8A, which is a schematic diagram illustrating an exemplary configuration of a multi-beam apparatus 300A consistent with embodiments of the present disclosure. A primary electron beam 102 generated by an electron source 101 is conditioned into three beamlets 102_1, 102_2, and 102_3 by three beamlet-forming apertures 172_1, 172_2, and 172_3 of a beamlet pre-forming mechanism 172. An anti-rotation condenser lens 110AR focuses the beamlets 102_1-102_3 to enter a source conversion unit 320.

[0062]

[0084] The source conversion unit 320 includes a beamlet pre-bending mechanism 123 having three pre-bending micro-deflectors 123_1, 123_2, 123_3, a beamlet limiting mechanism 121 having three beam limiting apertures 121_1 to 121_3, a beamlet compensation mechanism 322-2 having three micro-compensators 322-2_1, 322-2_2, 322-2_3, and one image forming mechanism 322-1 having three image forming micro-deflectors 322-1_1, 322-1_2, 322-1_3.

[0063]

[0085] The three pre-bending micro-deflectors 123_1 to 123_3 deflect the three beamlets 102_1 to 102_3 so that they are perpendicularly incident on the three beam-limiting apertures 121_1 to 121_3, respectively. The beam-limiting apertures 121_1 to 121_3 prevent the progression of residual peripheral electrons in the three beamlets 102_1 to 102_3, thus limiting their current. The three beamlets 102_1 to 102_3 are incident along their optical axes on the three micro-compensators 322-2_1 to 322-2_3, respectively. The three beamlets 102_1 to 102_3 then enter the three image-forming micro-deflectors 322-1_1 to 322-1_3, respectively, along their optical axes. The image forming micro-deflectors 322-1_1 to 322-1_3 deflect the beamlets 102_1 to 102_3 toward the main optical axis 300_1 of the apparatus 300A to form three virtual images of the electron source 101.

[0064]

[0086] The objective lens 131 focuses the three deflected beamlets 102_1 to 102_3 onto the surface 7 of the sample 8 under observation or inspection (i.e., projects three virtual images onto the surface 7). The three images formed on the surface 7 by the beamlets 102_1 to 102_3 form three probe spots 102_1S, 102_2S, and 102_3S on the surface. The deflection angles of the deflected beamlets 102_1 to 102_3 are adjusted to reduce off-axis aberrations of the three probe spots 102_1S to 102_3S caused by the objective lens 131, so that the three deflected beamlets pass through or approach the pre-focal point of the objective lens 131. The micro-compensators 322-2_1 to 322-2_3 are adjusted to compensate for residual field curvature and astigmatism of the probe spots 102_1S to 102_3S. The image forming mechanism 322-1 may further include an auxiliary micro-compensator for aberration compensation, which functions in conjunction with the beamlet compensation mechanism 322-2.

[0065]

[0087] Reference is now made to FIGS. 8B and 8C, which are cross-sectional views of the beamlet pre-forming mechanism 172 of FIG. 8A in the XY plane, consistent with an embodiment of the present disclosure. Adjusting the focusing power of the anti-rotation condenser lens 110AR allows the probe currents of the probe spots 102_1S-102_3S to be changed. FIG. 8B illustrates the sizes and positions of the three beamlets 102_1-102_3 for two focusing power settings. The focusing power is stronger in the second setting than in the first setting. In the first setting, only electrons within the small circle marks 102_1-102_3 can pass through the three beam-limiting apertures 121_1-121_3. In the second setting, only electrons within the large circle marks 102_1-102_3 can pass through the three beam-limiting apertures 121_1-121_3. Therefore, the probe currents of the three probe spots 102_1S-102_3S are greater in the second setting than in the first setting. The anti-rotation condenser lens 110AR keeps the rotation angles of the beamlets 102_1 to 102_3 unchanged for the two settings, so that the circle marks of the beamlets 102_1 to 102_3 move only in the radial direction.

[0066]

[0088] The shapes and sizes of the beamlet-forming apertures 172_1-172_3 are configured to cover the area covered by the circle marks of the beamlets 102_1-102_3 when the focusing power is adjusted within two settings (maximum focusing power, minimum focusing power). The shape of each beamlet-forming aperture can be configured to make its size as small as possible, thereby correspondingly reducing the Coulomb effect. In FIG. 8B, the beamlet-forming apertures 172_1-172_3 are round in shape, and the central beamlet-forming aperture 172_1 is smaller than the edge beamlet-forming apertures 172_2, 172_3. The shapes of the beamlet-forming apertures can be different for the same focusing power setting in FIG. 8B, such as those shown in FIG. 8C. 8C, the central beamlet-forming aperture 172_1 is round, the left edge beamlet-forming aperture 172_2 is polygonal, and the right edge beamlet-forming aperture 172_3 is elliptical. According to the same principle, the size and shape of the beamlet-forming apertures can be arbitrary. The size and shape of the beamlet-forming apertures disclosed in the embodiments are for illustrative purposes only and are not limiting.

[0067]

[0089] Reference is now made to FIG. 9A, which is a schematic diagram illustrating an exemplary configuration of a multi-beam apparatus 400A consistent with an embodiment of the present disclosure. In this embodiment, the beamlet-forming apertures 472_1, 472_2, and 472_3 of the beamlet pre-forming mechanism 472 split the primary electron beam 102 into three beamlets 102_1 through 102_3 while functioning as beam-limiting apertures to limit the beamlet current. Accordingly, compared to FIG. 8A, all unused electrons are blocked much earlier, thus further significantly reducing the Coulomb effect. Electrons of the beamlets 102_1 through 102_3 that strike the edges of the beamlet-forming apertures 472_1 through 472_3 may generate scattered electrons therefrom. These scattered electrons deviate from the normal paths of the beamlets 102_1 through 102_3 and contribute to background noise in the images generated by the beamlets 102_1 through 102_3. In the source conversion unit 420, the beam limiting openings 421_1, 421_2, 421_3 of the beamlet limiting mechanism 421 function as contrast apertures that block the progression of scattered electrons. The beamlet limiting mechanism 421 can also be located between the beamlet compensation mechanism 322-2 and the image forming mechanism 322-1.

[0068]

[0090] The probe currents of the three probe spots 102_1S-102_3S can be varied by changing the sizes of the beamlet-forming apertures 472_1-472_3. To change the sizes, the movable beamlet pre-forming mechanism 472 can be configured to be movable and to have two or more aperture groups. The size of the apertures in one group can be different from the size of the apertures in another group. The beamlet pre-forming mechanism 472 can be moved to set the apertures in one aperture group to act as beamlet-forming apertures.

[0069]

[0091] Reference is now made to Figure 9B, which is a schematic diagram illustrating an example configuration of the movable beamlet pre-forming mechanism 472 of Figure 9A consistent with an embodiment of the present disclosure. In Figure 9B, the movable beamlet pre-forming mechanism 472 has two aperture groups. The size and spacing of apertures 472_1-1, 472_2-1, and 472_3-1 in group 472-1 are configured to be smaller than those of apertures 472_1-2, 472_2-2, and 472_3-2 in group 472-2. Therefore, the probe current provided by group 472-1 is smaller than that of group 472-2.

[0070]

[0092] When group 472-1 is selected, the beamlet pre-forming mechanism 472 is moved so that apertures 472_1-1 to 472_3-1 can split the primary electron beam 102 into beamlets 102_1 to 102_3. The corresponding paths of the beamlets 102_1 to 102_3 are shown in FIG. 9A. When group 472-2 is selected, the beamlet pre-forming mechanism 472 is moved so that apertures 472_1-2 to 472_3-2 can split the primary electron beam 102 into beamlets 102_1 to 102_3. The corresponding paths of the beamlets 102_1 to 102_3 are shown in FIG. 9C.

[0071]

[0093] The beamlets 102_1-102_3 in FIG. 9C are more tightly focused than in FIG. 9A as they enter the pre-bend micro-deflectors 123_1-123_3 of the beamlet pre-bend mechanism 123. The anti-rotation focusing lens 110AR keeps the rotation angles of the beamlets 102_1-102_3 unchanged when different aperture groups are used. Thus, the aperture groups can be placed side-by-side, and the beamlet pre-forming mechanism 472 does not need to rotate when changing aperture groups. The non-rotation simplifies the structure of the beamlet pre-forming mechanism 472.

[0072]

[0094] In a multi-beam device using a movable condenser lens for varying the current of multiple beamlets and a beamlet pre-forming mechanism for reducing the Coulomb effect, such as that shown in FIG. 1B, the Coulomb effect can be further reduced if the movable condenser lens is configured to become a movable anti-rotation condenser lens using the movable anti-rotation condenser lens described above. The movable anti-rotation condenser lens can keep the rotation angles of the multiple beamlets unchanged or substantially unchanged when varying their currents, thereby eliminating the mismatch between the multiple beamlets and the corresponding beam-limiting apertures. The rotation angles of the multiple beamlets can be viewed as substantially unchanged if the change is 6° or less. Accordingly, the size of the beamlet-forming aperture of the beamlet pre-forming mechanism does not need to be enlarged to cover the mismatch, thereby preventing more unused electrons from passing through.

[0073]

[0095] Reference is now made to Figure 10, which is a schematic diagram illustrating an exemplary configuration of a multi-beam apparatus 500A consistent with embodiments of the present disclosure. In these embodiments, a primary electron beam 102 generated by an electron source 101 is conditioned into three beamlets 102_1, 102_2, and 102_3 by three beamlet-forming apertures 172_1, 172_2, and 172_3 of a beamlet pre-forming mechanism 172. A movable anti-rotation condenser lens 210AR focuses the beamlets 102_1-102_3 for normal incidence on the source conversion unit 520.

[0074]

[0096] The source conversion unit 520 includes a beamlet limiting mechanism 121 having three beam-limiting apertures 121_1 to 121_3, a beamlet compensation mechanism 322-2 having three micro-compensators 322-2_1, 322-2_2, and 322-2_3, and an image forming mechanism 322-1 having three image forming micro-deflectors 322-1_1, 322-1_2, and 322-1_3. The beam-limiting apertures 121_1 to 121_3 prevent the progression of residual peripheral electrons in the three beamlets 102_1 to 102_3, thus limiting their current. The three beamlets 102_1 to 102_3 are incident on the three micro-compensators 322-2_1 to 322-2_3, respectively, along their optical axes. The three beamlets 102_1 to 102_3 then enter the three image-forming micro-deflectors 322-1_1 to 322-1_3 along their optical axes, respectively. The image-forming micro-deflectors 322-1_1 to 322-1_3 deflect the beamlets 102_1 to 102_3 toward the main optical axis 500_1 of the device 500, forming three virtual images of the electron source 101.

[0075]

[0097] The objective lens 131 focuses the three deflected beamlets 102_1 to 102_3 onto the surface 7 of the sample 8 under observation (i.e., projects three virtual images onto the surface 7). The images formed on the surface 7 by the beamlets 102_1 to 102_3 form three probe spots 102_1S, 102_2S, and 102_3S on the surface.

[0076]

[0098] The deflection angles of the deflected beamlets 102_1 to 102_3 are adjusted to reduce off-axis aberrations of the three probe spots 102_1S to 102_3S caused by the objective lens 131, so that the three deflected beamlets pass through or approach the front focus of the objective lens 131. The micro-compensators 322-2_1 to 322-2_3 are adjusted to compensate for residual field curvature and astigmatism of the probe spots 102_1S to 102_3S. The imaging mechanism 322-1 may further include an auxiliary micro-compensator for aberration compensation, which functions together with the beamlet compensation mechanism 322-2.

[0077]

[0099] By adjusting the focusing power of the movable anti-rotation condenser lens 210AR and moving the position of its first principal plane 210AR_2 accordingly, the probe currents of the probe spots 102_1S-102_3S can be changed while keeping the beamlets 102_1-102_3 perpendicularly incident on the source transformation unit 520, with an unchanged or substantially unchanged rotation angle. FIG. 8B can also be used to illustrate the size and position of the three beamlets 102_1-102_3 in the beamlet pre-forming mechanism 172 for two exemplary settings of the focusing power and first principal plane 210AR_2. The focusing power in the second setting is stronger than in the first setting, and the position of the first principal plane in the second setting is closer to the electron source 101 than in the first setting. For the first setting, only electrons within the small circle marks 102_1-102_3 can pass through the three beam-limiting apertures 121_1-121_3. In the second setting, only electrons within the large circle marks 102_1 to 102_3 can pass through the three beam-limiting apertures 121_1 to 121_3. Therefore, the probe currents of the three probe spots 102_1S to 102_3S are larger in the second setting than in the first setting. Because the anti-rotation condenser lens 210AR can keep the rotation angles of the beamlets 102_1 to 102_3 unchanged for the two settings, the circle marks of the beamlets 102_1 to 102_3 move only in the radial direction.

[0078]

[0100] The shape and size of the beamlet-forming apertures 172_1-172_3 are configured to cover the area covered by the circle marks of the beamlets 102_1-102_3 when the movable anti-rotation focusing lens 210AR is adjusted within two settings. The shape of each beamlet-forming aperture can be configured to reduce its size, so that the Coulomb effect can be correspondingly reduced. It is understood that the size of each beamlet-forming aperture can be reduced as much as possible to correspondingly reduce the Coulomb effect as much as possible. Therefore, the shapes of the beamlet-forming apertures can be the same as each other (e.g., as shown in FIG. 8B) or different from each other (e.g., as shown in FIG. 8C). The beamlet-forming apertures 172_1-172_3 can be configured as a circle, an ellipse, a polygon, or any other shape.

[0079]

[0101] Reference is now made to FIG. 11A, which is a schematic diagram illustrating an exemplary configuration of a multi-beam apparatus 600A consistent with an embodiment of the present disclosure. In this embodiment, the beamlet-forming apertures 672_1, 672_2, and 672_3 of the beamlet pre-forming mechanism 672 split the primary electron beam 102 into three beamlets 102_1 through 102_3 while functioning as beam-limiting apertures to limit the beamlet current. Accordingly, compared to FIG. 10, all unused electrons are blocked much earlier, thus further significantly reducing the Coulomb effect. Electrons of the beamlets 102_1 through 102_3 that strike the edges of the beamlet-forming apertures 672_1 through 672_3 may generate scattered electrons therefrom. These scattered electrons deviate from the normal paths of the beamlets 102_1 through 102_3 and contribute to background noise in the images generated by the beamlets 102_1 through 102_3. In the source conversion unit 620, the beam limiting openings 621_1, 621_2, 621_3 of the beamlet limiting mechanism 621 function as contrast apertures that block the progression of scattered electrons. The beamlet limiting mechanism 621 can also be located between the beamlet compensation mechanism 322-2 and the image forming mechanism 322-1.

[0080]

[0102] The probe currents of the three probe spots 102_1S-102_3S can be varied by changing the size of the beamlet-forming apertures 672_1-672_3. To change the size, the beamlet pre-forming mechanism 672 can be configured to be movable and to have two or more aperture groups. The size of the apertures in one group can be different from the size of the apertures in another group. The beamlet pre-forming mechanism 672 can be moved to set the apertures in one aperture group to act as beamlet-forming apertures.

[0081]

[0103] Reference is now made to FIG. 11B, which is a schematic diagram illustrating an exemplary configuration of the movable beamlet pre-forming mechanism 672 of FIG. 11A consistent with an embodiment of the present disclosure. In FIG. 11B, the movable beamlet pre-forming mechanism 672 has two aperture groups. The size and spacing of the apertures 672_1-1, 672_2-1, and 672_3-1 of group 672-1 are configured to be smaller than those of the apertures 672_1-2, 672_2-2, and 672_3-2 of group 672-2. The probe current provided by group 672-1 is smaller than that of group 672-2. When group 672-1 is selected, the beamlet pre-forming mechanism 672 is moved so that the apertures 672_1-1 through 672_3-1 can split the primary electron beam 102 into beamlets 102_1 through 102_3. The corresponding paths of beamlets 102_1 to 102_3 are shown in Figure 11A. When group 672-2 is selected, the beamlet pre-forming mechanism 672 is moved so that apertures 672_1-2 to 672_3-2 can split the primary electron beam 102 into beamlets 102_1 to 102_3. The corresponding paths of beamlets 102_1 to 102_3 are shown in Figure 11C.

[0082]

[0104] The beamlets 102_1-102_3 in Figure 11C are focused more strongly and earlier than in Figure 11A. The movable anti-rotation lens 210AR keeps the rotation angle unchanged when different aperture groups are used. Thus, the aperture groups can be placed side-by-side, and the beamlet pre-forming mechanism 672 does not need to rotate when changing aperture groups. The non-rotation simplifies the structure of the beamlet pre-forming mechanism 672.

[0083]

[0105] In the case of a multi-beam device, more beamlets are required to achieve higher throughput. To make more beamlets available, the beamlet spacing of the source conversion unit is configured as small as possible. In the case of a multi-beam device, a larger fluctuation range of the probe spot current is required to realize observation or inspection of more types of samples. Consequently, if the beamlet spacing is small enough for a certain fluctuation range of the probe current of a multi-beam device such as the embodiment 300A of FIG. 8A and the embodiment 500A of FIG. 10, some beamlet-forming apertures of the beamlet pre-forming mechanism 172 may partially overlap, as shown in FIGS. 12A and 12B. The partially overlapping beamlet-forming apertures can be configured to be one combined beamlet-forming aperture. Beamlets passing through two adjacent beam-limiting apertures of the source conversion unit can both pass through this combined beamlet-forming aperture.

[0084]

[0106] 12A and 12B, which are cross-sectional views in the XY plane of a beamlet pre-forming mechanism (e.g., beamlet pre-forming mechanism 172 of FIG. 8A) consistent with embodiments of the present disclosure. FIG. 12A illustrates an example having five beamlets 102_1, 102_2, 102_3, 102_4, and 102_5. The small circle marks for the beamlets 102_1 through 102_5 correspond to the beamlets 102_1 through 102_5 having small probe currents, and the large circle marks for the beamlets 102_1 through 102_5 correspond to the beamlets 102_1 through 102_5 having large probe currents. Beamlets 102_2 and 102_4 pass through combined beamlet-forming aperture 172_2+4, and beamlets 102_3 and 102_5 pass through combined beamlet-forming aperture 172_3+5. Similarly, the combined beamlet-forming apertures can be configured with different shapes and sizes to prevent unused electrons from passing through as much as possible. For example, combined beamlet-forming aperture 172_2+4 is elliptical, and combined beamlet-forming aperture 172_3+5 is polygonal. Figure 12B shows an example with 25 beamlets. Beamlet pre-forming mechanism 172 has four combined beamlet-forming apertures and 17 single beamlet-forming apertures.

[0085]

[0107] Reference is now made to Figures 13 and 14, which are each schematic diagrams illustrating an example configuration of a multi-beam device having the beamlet pre-forming mechanism of Figure 12A consistent with an embodiment of the present disclosure. The configuration of the multi-beam device 310A of Figure 13 is similar to the configuration of the multi-beam device 300A of Figure 8A. In the multi-beam device 310A of Figure 13, there are five beamlets 102_1, 102_2, 102_3, 102_4, and 102_5. The beamlet pre-forming mechanism 172 has three beamlet-forming apertures 172_1, 172_2+4, and 172_3+5, and the beamlet-forming apertures 172_2+4 and 172_3+5 are combined beamlet-forming apertures. The central beamlet 102_1 passes through the central beamlet forming aperture 172_1, the two left beamlets 102_2, 102_4 pass through the left beamlet forming aperture 172_2+4, and the two right beamlets 102_3, 102_5 pass through the right beamlet forming aperture 172_3+5.

[0086]

[0108] The configuration of the multi-beam device 510A in Figure 14 is similar to the configuration of the multi-beam device 500A in Figure 10A. In the configuration of the multi-beam device 510A in Figure 14, there are five beamlets 102_1, 102_2, 102_3, 102_4, and 102_5. The beamlet pre-forming mechanism 172 has three beamlet-forming apertures 172_1, 172_2+4, and 172_3+5, and the beamlet-forming apertures 172_2+4 and 172_3+5 are combined beamlet-forming apertures. The central beamlet 102_1 passes through the central beamlet forming aperture 172_1, the two left beamlets 102_2, 102_4 pass through the left beamlet forming aperture 172_2+4, and the two right beamlets 102_3, 102_5 pass through the right beamlet forming aperture 172_3+5.

[0087]

[0109] Returning to FIG. 1A, multiple probe spots 102_1S to 102_3S in a conventional device are deflected by a common deflection scanning unit to scan multiple small scanning areas. Therefore, the scanning characteristics (scanning direction, scanning range, scanning speed, etc.) of all beamlets are the same. However, for some samples under observation or inspection, the pattern features in different small scanning areas may be very different. To obtain better image contrast of the images generated by the multiple probe spots, the scanning characteristics of some or all of the beamlets need to be different and should be individually set according to the pattern features in each small scanning area. Each imaging micro-deflector of one source conversion unit mentioned above can perform individual deflection scanning.

[0088]

[0110] Reference is now made to Figures 15A-15C, which are schematic diagrams illustrating an exemplary configuration of a multi-beam device 700A with individual deflection scanning consistent with embodiments of the present disclosure. The configuration of the multi-beam device 700A in Figure 15A has some similarities to the multi-beam device 100A in Figure 1A, but the multi-beam device 700A further includes an image-forming mechanism 722 in the source conversion unit 720. In Figure 15A, each image-forming micro-deflector 722_1, 722_2, 722_3 in the image-forming mechanism 722 can deflect one of the beamlets 102_1, 102_2, 102_3 to form a virtual image of the electron source 101, and can additionally dynamically deflect one beamlet to scan the corresponding probe spot over a small scan area. Figure 15B illustrates exemplary scan paths of the probe spots 102_1S, 102_2S, 102_3S. Probe spot 102_1S scans area A1 having a dashed outline in the direction between the X-axis and Y-axis directions, probe spot 102_2S scans area A2 having a dashed outline in the X-axis direction, and probe spot 102_3S scans area A3 having a line outline in the Y-axis direction.

[0089]

[0111] In Figure 15A, secondary or signal beams 102_1se, 102_2se, and 102_3se are generated by probe spots 102_1S to 102_3S from scan regions A1, A2, and A3. A beam separator 160 deflects the secondary beams 102_1se to 102_3se to enter a secondary projection imaging system 150. The secondary projection imaging system 150 focuses the secondary beams 102_1se to 102_3se to be detected by detection elements 140_1, 140_2, and 140_3 of an electronic detection device 140M. Figure 15C shows an exemplary scanning path of the secondary beams 102_1se to 102_3se on the detection elements 140_1, 140_2, and 140_3. If the scanning path of each secondary beam exceeds the corresponding detection element, an anti-scan deflection unit (not shown in FIG. 15A) placed in front of the electronic detection device 140M can be used to at least partially stop the movement of the secondary beam on the detection elements 140_1 to 140_3.

[0090]

[0112] The following clauses may be used to further describe the embodiments. 1. An anti-rotation lens having focusing power for focusing a charged particle beam, comprising: a first magnetic lens configured to generate a first magnetic field and to be aligned with the optical axis of the anti-rotation lens; a second magnetic lens configured to generate a second magnetic field and to be aligned with the optical axis; An anti-rotation lens, wherein the focusing power of the anti-rotation lens is adjustable by varying a first magnetic field and a second magnetic field, the first magnetic field and the second magnetic field having opposite directions on the optical axis. 2. An anti-rotation lens as described in clause 1, wherein the focusing power is adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 3. An anti-rotation lens as described in clause 2, wherein the rotation angle is zero. 4. An anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: a magnetic lens configured to generate a magnetic field and to be aligned with the optical axis of the anti-rotation lens; an electrostatic lens configured to generate an electrostatic field and to be aligned with the optical axis; An anti-rotation lens wherein the magnetic and electrostatic fields at least partially overlap, and the focusing power of the anti-rotation lens is adjustable by varying the magnetic and / or electrostatic fields. 5. An anti-rotation lens as described in clause 5, wherein the focusing power is adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 6. An anti-rotation lens according to clause 5, wherein the rotation angle is zero. 7. A movable anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: a first magnetic lens configured to generate a first magnetic field and to be aligned with the optical axis of the movable anti-rotation lens; a second magnetic lens configured to generate a second magnetic field and to be aligned with the optical axis; a third magnetic lens configured to generate a third magnetic field and to be aligned with the optical axis; A movable anti-rotation lens, wherein the focusing power and principal plane of the movable anti-rotation lens are adjustable by varying a first magnetic field, a second magnetic field, and / or a third magnetic field, and two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis. 8. A movable anti-rotation lens according to clause 7, wherein the focusing power and the principal plane are adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 9. A movable anti-rotation lens as described in clause 8, having a zero rotation angle. 10. A movable anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: an anti-rotation lens configured to be aligned with the optical axis of the movable anti-rotation lens; a lens configured to be aligned with the optical axis; A movable anti-rotation lens, wherein the focusing power and the main plane of the movable anti-rotation lens are adjustable by changing the focusing power of the anti-rotation lens and / or the focusing power of the lens, and the main plane is adjustable relative to a source that generates a charged particle beam. 11. A movable anti-rotation lens according to clause 10, wherein the focusing power and principal plane are adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 12. A movable anti-rotation lens according to clause 11, having a rotation angle of zero. 13. A movable anti-rotation lens as described in clause 10, wherein the lens is an electrostatic lens. 14. A movable anti-rotation lens as described in clause 10, wherein the lens is a magnetic lens. 15. A movable anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: a first anti-rotation lens configured to be aligned with the optical axis of the movable anti-rotation lens; a second anti-rotation lens configured to be aligned with the optical axis; A movable anti-rotation lens, wherein the focusing power and the principal plane of the movable anti-rotation lens are adjustable by varying the focusing power of the first anti-rotation lens and / or the focusing power of the second anti-rotation lens. 16. A movable anti-rotation lens according to clause 15, wherein the focusing power and major plane of the movable anti-rotation lens are adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 17. A movable anti-rotation lens according to clause 16, having a zero rotation angle. 18. A multi-beam device for observing a sample, comprising: an electron source configured to generate a primary electron beam; a focusing lens configured to focus the primary electron beam, the focusing lens being one of an anti-rotation lens or a movable anti-rotation lens; a source conversion unit configured to form multiple images of the electron source with multiple beamlets of the primary electron beam; an objective lens configured to focus the multiple beamlets onto the surface to form multiple probe spots on the surface; an electronic detection device having multiple detection elements configured to detect multiple secondary beams generated by the multiple probe spots from the sample; A multi-beam device comprising: 19. A multi-beam apparatus according to clause 18, further comprising a beamlet pre-forming mechanism located between the electron source and the condenser lens and including a plurality of beamlet-forming apertures. 20. A multi-beam device according to clause 19, wherein the plurality of beamlet-forming apertures are configured to condition the primary electron beam into a plurality of beamlets. 21. A multi-beam device according to clause 20, wherein the focusing lens is configured to focus the multiple beamlets to be incident on the source transformation unit at multiple rotation angles. 22. A multi-beam apparatus according to clause 21, wherein the plurality of rotation angles remain unchanged or substantially unchanged when varying the probe current of the multiple probe spots. 23. A multi-beam device according to any one of clauses 20 to 22, wherein the plurality of beamlets constitutes a multiplicity of beamlets. 24. A multi-beam device according to any one of clauses 19 to 23, wherein the probe current can be varied by varying the size of the plurality of beamlet-forming apertures. 25. A multi-beam device according to clause 20, 23 or 24, wherein the source transformation unit comprises a number of beam-limiting apertures configured to condition the plurality of beamlets into a number of beamlets. 26. A multi-beam device according to any one of clauses 18 to 25, wherein the probe current can be changed by adjusting the focusing power of the focusing lens. 27. A multi-beam device according to any one of clauses 19 to 26, wherein the plurality of beamlet-forming apertures are configured to prevent the progression of electrons not within a number of probe spots. 28. A multi-beam device according to any one of clauses 18 to 27, wherein the source transformation unit comprises an image forming mechanism configured to deflect multiple beamlets to form multiple images. 29. A multi-beam device according to clause 28, wherein the image forming mechanism comprises a number of electron optical elements configured to deflect a number of beamlets to form a number of images. 30. A multi-beam device according to clause 28 or 29, wherein the deflection angles of the multiple beamlets are set individually to reduce aberrations of the multiple probe spots. 31. A multi-beam device according to clause 29, wherein the multiple electron optical elements are configured to compensate for off-axis aberrations of the multiple probe spots. 32. A multi-beam device according to any one of clauses 18 to 31, wherein the source transformation unit comprises a beamlet compensation mechanism configured to compensate for off-axis aberrations of the multiple probe spots. 33. A multi-beam apparatus according to clause 32, wherein multiple electron optical elements and beamlet compensation mechanisms together compensate for aberrations of multiple probe spots. 34. A multi-beam device according to clause 29, further comprising a deflection scanning unit positioned below the source conversion unit. 35. A multi-beam apparatus according to clause 34, wherein the deflection scanning unit is configured to deflect multiple beamlets to scan multiple probe spots. 36. A multi-beam device according to clause 34 or 35, wherein multiple electron optical elements are configured to deflect multiple beamlets to scan multiple probe spots. 37. A multi-beam apparatus according to clause 34 or 35, wherein the deflection scanning unit and the multiple electron optical elements are together configured to deflect multiple beamlets to scan multiple probe spots. 38. A multi-beam device according to clause 29, wherein multiple electron optical elements are configured to deflect multiple beamlets to scan multiple probe spots. 39. A multi-beam device according to any one of clauses 36 to 38, wherein one or more of the multiple probe spots may differ in one or more scanning characteristics. 40. A multi-beam device according to clause 39, wherein one of the scanning characteristics comprises a scanning direction. 41. A multi-beam device according to clause 39, wherein one of the scanning characteristics comprises a scan size. 42. A multi-beam device as described in clause 39, wherein one of the scanning characteristics includes a scanning speed. 43. A multi-beam apparatus according to any one of clauses 36 to 39, further comprising an anti-scan deflection unit positioned before the electronic detection device and configured to deflect the multiple secondary beams towards the multiple detection elements. 44. A method for constructing an anti-rotation lens for focusing a charged particle beam, comprising: generating a first magnetic field with a first magnetic lens aligned with the optical axis of the anti-rotation lens; generating a second magnetic field by a second magnetic lens aligned with the optical axis; generating a focusing force of the anti-rotation lens by a first magnetic field and a second magnetic field, the first magnetic field and the second magnetic field having opposite directions on the optical axis; A method comprising: 45. The method of clause 44, further comprising varying the focusing force by adjusting the first magnetic field and the second magnetic field while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 46. ​​A method for constructing an anti-rotation lens for focusing a charged particle beam, comprising: generating a magnetic field by a magnetic lens; generating an electrostatic field by an electrostatic lens; generating a focusing force of the anti-rotation lens by magnetic and / or electrostatic fields, the magnetic and electrostatic fields at least partially overlapping; A method comprising: 47. The method of clause 46, further comprising varying the focusing force by adjusting the magnetic and / or electrostatic fields while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 48. A method for constructing a movable anti-rotation lens for focusing a charged particle beam, comprising: generating a first magnetic field with a first magnetic lens aligned with an optical axis of the movable anti-rotation lens; generating a second magnetic field by a second magnetic lens aligned with the optical axis; generating a third magnetic field by a third magnetic lens aligned with the optical axis; generating a focusing force of the movable anti-rotation lens by a first magnetic field, a second magnetic field, and / or a third magnetic field, wherein two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis; A method comprising: 49. The method of clause 48, further comprising varying the focusing power and moving the major plane of the movable anti-rotation lens by adjusting the first magnetic field, the second magnetic field and / or the third magnetic field while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged. 50. A method for constructing a multi-beam device for observing a sample, comprising: conditioning a primary electron beam from the electron source into a plurality of beamlets by a beamlet pre-forming mechanism positioned between the electron source and the condenser lens; forming multiple images of the electron source using a plurality of beamlets with a source conversion unit; forming multiple probe spots on the sample by projecting multiple images onto the sample; adjusting a collection lens to keep the rotation angle of the plurality of beamlets unchanged or substantially unchanged when changing the probe current of the multiple probe spots, the collection lens being one of an anti-rotation lens or a movable anti-rotation lens; A method comprising: 51. The method of clause 50, further comprising adjusting the source conversion unit to scan multiple probe spots on the sample. 52. A non-transitory computer-readable medium storing a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring an anti-rotation lens for focusing a charged particle beam, the method comprising: directing a first magnetic lens to generate a first magnetic field, the first magnetic lens being aligned with an optical axis of the anti-rotation lens; directing a second magnetic lens to generate a second magnetic field, the second magnetic lens being aligned with the optical axis; the first magnetic field and the second magnetic field generate a focusing force of the anti-rotation lens; A non-transitory computer-readable medium, wherein the first magnetic field and the second magnetic field have opposite directions on an optical axis. 53. A multi-beam device comprising: a set of instructions executable by one or more processors; The non-transitory computer readable medium of clause 52, further causing the multi-beam device to adjust the first magnetic field and the second magnetic field while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged to adjust the focusing force. 54. A non-transitory computer-readable medium storing a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring an anti-rotation lens for focusing a charged particle beam, the method comprising: directing a magnetic lens to generate a magnetic field; directing the electrostatic lens to generate an electrostatic field; a magnetic and / or electrostatic field generates the focusing force of the anti-rotation lens; A non-transitory computer-readable medium in which a magnetic field and an electrostatic field at least partially overlap. 55. A non-transitory computer readable medium as described in clause 54, wherein a set of instructions executable by one or more processors of the multi-beam device further causes the multi-beam device to execute. 56. Adjusting the magnetic and / or electrostatic fields while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged to change the focusing force. 57. A non-transitory computer-readable medium storing a set of instructions executable by one or more processors of a multi-beam device to cause the multi-beam device to perform a method for configuring a movable anti-rotation lens for focusing a charged particle beam, the method comprising: directing a first magnetic lens to generate a first magnetic field, the first magnetic lens being aligned with an optical axis of the movable anti-rotation lens; directing a second magnetic lens to generate a second magnetic field, the second magnetic lens being aligned with the optical axis; directing a third magnetic lens to generate a third magnetic field, wherein the third magnetic lens is aligned with the optical axis; the first magnetic field, the second magnetic field, and / or the third magnetic field generate a focusing force of the movable anti-rotation lens; 10. A non-transitory computer-readable medium, wherein two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on an optical axis. 58. A multi-beam device comprising: a set of instructions executable by one or more processors; 58. The non-transitory computer readable medium of clause 57, further causing the multi-beam device to adjust the first magnetic field, the second magnetic field and / or the third magnetic field while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged to change the focusing power and move the major plane of the movable anti-rotation lens.

[0091]

[0113] It is understood that the controller of the multi-beam device can use software to control the functionality described above. For example, the controller can send instructions to the aforementioned lenses to generate appropriate fields (e.g., magnetic or electrostatic fields). The software can be stored on a non-transitory computer-readable medium. Common forms of non-transitory medium include, for example, floppy disks, flexible disks, hard disks, solid-state drives, magnetic tape, any other magnetic data storage medium, CD-ROMs, any other optical data storage medium, any physical medium with a pattern of holes, RAM, PROM, EPROM, flash EPROM, any other flash memory, NVRAM, cache, registers, any other memory chip or cartridge, and network-connected versions thereof.

[0092]

[0114] Although the present invention has been described in connection with preferred embodiments thereof, it should be understood that other changes and modifications can be made without departing from the spirit and scope of the invention as hereinafter claimed.

Claims

1. 1. An anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: a first magnetic lens aligned with the optical axis of the anti-rotation lens to generate a first magnetic field; a second magnetic lens aligned with the optical axis to generate a second magnetic field; the focusing power of the anti-rotation lens is adjustable by varying the first magnetic field and the second magnetic field; An anti-rotation lens, wherein the first magnetic field and the second magnetic field have opposite directions on the optical axis.

2. The anti-rotation lens of claim 1 , wherein the focusing force is adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged.

3. The anti-rotation lens of claim 2 , wherein the rotation angle is zero.

4. the anti-rotation lens includes a third magnetic lens aligned with the optical axis that generates a third magnetic field; the focusing power and principal plane of the anti-rotation lens are adjustable by varying the first magnetic field, the second magnetic field, and / or the third magnetic field; The anti-rotation lens of claim 1 , wherein two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis.

5. 5. The anti-rotation lens of claim 4, wherein the focusing power and the major plane are adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged.

6. 1. An anti-rotation lens having a focusing power for focusing a charged particle beam, comprising: a magnetic lens aligned with the optical axis of the anti-rotation lens and generating a magnetic field; an electrostatic lens aligned with the optical axis to generate an electrostatic field; the magnetic field and the electrostatic field at least partially overlap; An anti-rotation lens, wherein the focusing power of the anti-rotation lens is adjustable by varying the magnetic field and / or the electrostatic field.

7. 7. The anti-rotation lens of claim 6, wherein the focusing force is adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged.

8. the anti-rotation lens further comprises a lens aligned with the optical axis; the focusing power and principal plane of the anti-rotation lens are adjustable by varying the magnetic field of the magnetic lens, the electrostatic field of the electrostatic lens, and / or the focusing power of the lens; The anti-rotation lens of claim 6 , wherein the major plane is adjustable relative to a source that generates the charged particle beam.

9. 9. The anti-rotation lens of claim 8, wherein the focusing power and the major plane are adjustable while keeping the rotation angle of the charged particle beam unchanged or substantially unchanged.

10. The anti-rotation lens of claim 8, wherein the lens is an electrostatic lens or the lens is a magnetic lens.

11. an electron source for generating a primary electron beam; a condenser lens that focuses the primary electron beam; a source conversion unit for forming multiple images of the electron source with multiple beamlets of the primary electron beam; an objective lens that focuses the multiple beamlets onto a surface to form multiple probe spots on the surface; A multi-beam device comprising: A multi-beam device further comprising the anti-rotation lens of claim 1.

12. 12. The multi-beam apparatus of claim 11, further comprising an electronic detection device having multiple detection elements for detecting multiple secondary beams generated by the multiple probe spots from the sample for observing the sample.

13. an electron source for generating a primary electron beam; a condenser lens that focuses the primary electron beam; a source conversion unit for forming multiple images of the electron source with multiple beamlets of the primary electron beam; an objective lens that focuses the multiple beamlets onto a surface to form multiple probe spots on the surface; A multi-beam device comprising: A multi-beam device further comprising the anti-rotation lens of claim 6.

14. 1. A method for constructing an anti-rotation lens for focusing a charged particle beam, comprising: generating a first magnetic field with a first magnetic lens aligned with an optical axis of the anti-rotation lens; generating a second magnetic field with a second magnetic lens aligned with the optical axis; generating a focusing force of the anti-rotation lens with the first magnetic field and the second magnetic field, the first magnetic field and the second magnetic field having opposite directions on the optical axis; The method further comprising adjusting the focusing force by adjusting the first magnetic field and the second magnetic field while keeping a rotation angle of the charged particle beam unchanged or substantially unchanged.

15. 1. A method for constructing an anti-rotation lens for focusing a charged particle beam, comprising: generating a magnetic field by a magnetic lens; generating an electrostatic field by an electrostatic lens; generating a focusing force of the anti-rotation lens by the magnetic field and / or the electrostatic field, wherein the magnetic field and the electrostatic field at least partially overlap; The method further comprising adjusting the focusing force by adjusting the magnetic field and / or the electrostatic field while keeping a rotation angle of the charged particle beam unchanged or substantially unchanged.

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