Counter-pole with permanent magnet

Abstract

Counter pole with permanent magnets. A charged particle beam system can include a vacuum chamber, a specimen holder for fixing a specimen within the vacuum chamber, and a charged particle column. The charged particle column can include a charged particle source for generating a charged particle beam along an optical axis and a magnetic immersion lens for focusing the charged particle beam. The magnetic immersion lens can include a first lens pole disposed adjacent to a first surface of the specimen, an excitation coil surrounding the first lens pole, and a counter pole disposed adjacent to a second surface of the specimen, the counter pole including one or more magnets disposed on the counter pole.

Description

Technical Field

[0001] This disclosure relates to apparatus and methods for improving magnetic immersion lenses. Background Technology

[0002] Charged particle beam systems are used in a variety of applications, including the fabrication, repair, and inspection of microdevices such as integrated circuits, magnetic recording heads, and photolithographic masks. Dual-beam systems typically comprise: a scanning electron microscope (SEM), which provides high-resolution images with minimal damage to the target; and an ion beam system, such as a focusing or shaping beam system, which can be used to modify the substrate (e.g., by milling) and form an image.

[0003] Generally, the final lens for SEM is a charged particle beam (CBP) lens that generates a magnetic field (e.g., a magnetic immersion lens). The strength of this immersion lens is limited by magnetic circuit saturation and the magnetic flux source (e.g., a coil). Magnetic saturation is the state reached when continuing to increase the applied external magnetic field can no longer further increase the magnetization of the material, and thus the total magnetic flux density flattens out. Therefore, there is a continuous need for improved immersion lenses. Summary of the Invention

[0004] In a representative embodiment, the charged particle beam system may include: a vacuum chamber; a sample holder for securing a sample within the vacuum chamber; and a charged particle column including a charged particle source for generating a charged particle beam along an optical axis and a magnetically immersed lens for focusing the charged particle beam. The magnetically immersed lens may include: a first lens pole disposed adjacent to a first surface of the sample; an excitation coil surrounding the first lens pole; and a reverse pole configured adjacent to a second surface of the sample, the reverse pole including one or more magnets disposed on the surface of the reverse pole.

[0005] In some embodiments, the surface of the reverse pole is a first surface, and one or more additional magnets are disposed on the second surface of the reverse pole.

[0006] In some embodiments, the reverse pole includes one or more notches, wherein each magnet is disposed within a respective notch. In some embodiments, the notch includes a track, and one or more magnets are slidable within the track, such that the magnets are positioned relative to the optical axis. The track may extend radially inward toward the optical axis from the outer edge of the reverse pole and / or may extend circumferentially around the optical axis. In other embodiments, one or more magnets may be attached to the reverse pole via an adhesive.

[0007] The reverse electrode can be mounted to a positioning system configured to allow the reverse electrode to move between a first position and a second position within a vacuum chamber, wherein the reverse electrode is inactive in either the first or second position. The reverse electrode may further include an aperture and a detector disposed within or below the aperture.

[0008] In some embodiments, the magnet may be configured to compensate for asymmetries in the system. In other embodiments, the magnet may be configured to generate asymmetries in the system.

[0009] In another representative embodiment, the system may be a dual-beam system, comprising: a sample holder for fixing a sample; an ion beam column configured to guide an ion beam to the sample; and a charged particle column including a charged particle source for generating a charged particle beam along an optical axis and a magnetic immersion lens for focusing the charged particle beam. The magnetic immersion lens may include: a first lens pole disposed adjacent to a first surface of the sample; and a second lens pole disposed adjacent to a second surface of the sample, the second lens pole including one or more magnets. In some embodiments, the magnetic immersion lens may be coupled to a control unit.

[0010] One method using a dual-beam system may include arranging magnets around the optical axis to create asymmetry within the system.

[0011] The foregoing and other objectives, features and advantages of this disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. Attached Figure Description

[0012] Figure 1 Describe a representative dual-beam system.

[0013] Figure 2 The description includes a representative SEM system with magnetic immersion objectives.

[0014] Figure 3 This is a top plan view of an embodiment for use as the reverse pole in a SEM system.

[0015] Figure 4 This is a top plan view of another embodiment of the reverse pole for a SEM system.

[0016] Figure 5 A perspective view of a portion of a representative SEM system that includes multiple permanent magnets.

[0017] Figure 6 A graph of the magnetic field as part of an exemplary SEM system.

[0018] Figure 7 A graph of the magnetic field as part of an exemplary SEM system comprising one or more permanent magnets. Detailed Implementation

[0019] introduction Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic class of electron microscopes has evolved into many well-known instrument types, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM), as well as various subclasses, such as so-called "dual-beam" tools (e.g., FIB-SEM), which additionally employ "processing" focused ion beams (FIB) to allow for supporting activities such as ion beam milling or ion beam induced deposition (IBID).

[0020] In SEM, irradiating a substrate with a scanning electron beam produces "auxiliary" radiation from the substrate in the form of secondary electrons, backscattered electrons, transmitted electrons, X-rays, and cathodoluminescence (infrared, visible, and / or ultraviolet photons). For example, one or more components of this auxiliary radiation can be detected and used for imaging. As an alternative to using electrons as the irradiation beam, charged particle microscopy can also be performed using other types of charged particles. In this regard, the phrase "charged particles" should be interpreted broadly to encompass, for example, electrons, positive ions (e.g., Ga or Xe ions), negative ions, protons, and positrons. In addition to imaging and performing localized surface modifications (e.g., milling, etching, deposition, etc.), charged particle microscopy can also have other functionalities, such as performing spectroscopy, examining diffraction patterns, etc.

[0021] In some instances, ion beam components are used to mill associated spots on a test substrate such as a silicon substrate. As used herein, “image” refers to a visual image and its digital or other stored representation.

[0022] Generally, the final lens for SEM is a charged particle beam (CBP) lens, such as an electrostatic, magnetic, or combined lens. In the case of a magnetically immersed lens, the strength and imaging quality of this immersion lens are typically limited by the saturation of the magnetic circuit and the magnetic flux source (e.g., a coil). Permanent magnets can be used in conjunction with the coil of a CBP lens to add additional flux to the magnetic circuit and / or introduce or compensate for asymmetries in the system, thereby improving the lens resolution.

[0023] As used herein, unless the context clearly indicates otherwise, the singular form “a / an” and “the” include the plural form. Additionally, the term “comprising” means “including”. Furthermore, the term “joining” does not necessarily exclude the existence of intermediate elements between the joined items.

[0024] The systems, apparatuses, and methods described herein should not be construed as limiting in any way. Rather, this disclosure relates to all novel and non-obvious features and aspects of the various disclosed embodiments (individually and in various combinations and sub-combinations with each other). The disclosed systems, methods, and apparatuses are not limited to any particular aspect or feature or combination thereof, nor are they required to have any one or more particular advantages or problems solved. Any operational theory is provided for ease of explanation, but the systems, methods, and apparatuses of this disclosure are not limited to such operational theories.

[0025] While some of the disclosed methods are described in a specific order for ease of presentation, it should be understood that this descriptive approach encompasses rearrangement unless the specific language described below requires a particular order. For example, in some cases, the operations described sequentially may be rearranged or performed simultaneously. Furthermore, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, this specification sometimes uses terms such as “produce” and “provide” to describe the disclosed methods. These terms are highly abstract representations of the actual operations performed. The actual operations corresponding to these terms will vary depending on the specific implementation and are readily discernible to those skilled in the art. In some instances, values, processes, or apparatus are referred to as “lowest,” “most preferred,” “smallest,” etc. It should be understood that such descriptions are intended to indicate that a selection can be made from many functional alternatives used, and that such selection is not necessarily better, smaller, or otherwise more preferred than other selections. References are used to describe examples of directions such as “above,” “below,” “upper,” “lower,” etc. These terms are used for convenience of description but do not imply any particular spatial orientation.

[0026] Example 1 Figure 1This illustration describes a representative embodiment of a dual-beam system 100 including a scanning electron microscope (SEM) 102 and an ion beam column 104. The SEM 102 may include one or more charged particle beam (CPB) lenses, such as a condenser lens 116 and an objective lens 106. In some embodiments, the one or more CPB lenses may be magnetic lenses, and specifically, the objective lens 106 may be a magnetic objective (e.g., a magnetic immersion objective). The ion beam column is arranged to provide a focused ion beam (FIB) to a sample S, and the SEM 102 is positioned to produce an image of the sample S. The SEM 102 and the ion beam column 104 may be mounted to a vacuum chamber 108, which houses a movable substrate holder 110 for securing the sample S. The vacuum chamber 108 may be evacuated using a vacuum pump (not shown). The substrate holder 110 may be movable in the XY plane as shown relative to coordinate system 150, where the Y-axis is perpendicular to the image plane. The substrate holder may be further moved vertically (along the Z-axis) to compensate for changes in the height of the sample S.

[0027] In some embodiments, the SEM 102 can be vertically arranged above the sample S and can be used to image the sample S, and the ion beam column 104 can be arranged at an angle and can be used to process and / or treat the sample S. Figure 1 Demonstration orientation of SEM 102 and ion beam column 104 is shown.

[0028] SEM 102 may include an electron source 112 and may be configured to manipulate the “raw” radiation beam from the electron source 112 and perform operations such as focusing, aberration reduction, cropping (using aperture), filtering, etc. SEM 102 may generate an input beam of charged particles 114 (e.g., an electron beam) propagating along the particle optical axis 115. One or more CPB lenses of SEM column 102, such as condenser lens 116 and objective lens 106, focus the beam 114 onto the sample S. In some embodiments, SEM 102 may be equipped with a deflection unit 118, which may be configured to manipulate the beam 114. For example, the beam 114 may be manipulated across the sample under study using scanning motion (e.g., raster or vector scan).

[0029] In some embodiments, such as Figure 1 As shown, the dual-beam system 100 may further include a magnetic field generating coil 107, which is configured to provide an additional magnetic field to generate an additional immersion magnetic lens, thereby improving the resolution of the SEM optical system.

[0030] The dual-beam system 100 may further include a computer processing device and / or a control unit 128, which is used, in particular, to control the deflection unit 118, CPB lenses 106, 116 and detectors (not shown), and to display information collected from the detectors on a display unit. In some cases, a control computer 130 is provided to establish various excitations, record imaging data and generally control the operation of both the SEM and FIB.

[0031] The ion beam post 104 may include an ion source (e.g., a plasma source 120) and ion beam optics 122. In the illustrated embodiment, the ion beam post 104 is a plasma focused ion beam (PFIB); however, in other embodiments, the ion beam post 104 may be a standard focused ion beam (FIB) with a liquid metal ion source (LMIS) or any other ion source compatible with a focused ion beam post. The ion beam post 104 may generate and / or guide the ion beam 124 along the ion optical axis 125. As mentioned above, the ion post 104 may be used to perform imaging, processing, and / or fabrication operations on a substrate, such as cutting, milling, etching, deposition, etc.

[0032] In embodiments where the ion beam is a PFIB, the ion source 120 can be fluidly coupled to a variety of gases via a gas manifold 126, the gas manifold including gas sources 142A to 142D coupled to the ion source 120 via corresponding valves 141A to 141D. Valve 140 is positioned to selectively couple gas from the gas manifold 126 to the ion source 120. Figure 1 As shown, the exemplary gases include, but are not limited to, xenon, argon, oxygen, and nitrogen. During operation of the ion source 120, gases may be introduced, which become charged or ionized, thereby forming a plasma. Subsequently, ions extracted from the plasma can be accelerated through the ion beam column 104, thus forming an ion beam.

[0033] Example 2 In another representative embodiment, system 200 may include a scanning electron microscope (SEM) 202 mounted on a vacuum chamber 204. The vacuum chamber 204 may house a first movable stage 206 for fixing a sample S. The sample S may be introduced via a load lock 208. The vacuum chamber 202 may be evacuated using a vacuum pump (not shown).

[0034] SEM 202 may include one or more CPB lenses, such as condenser lens 210 and objective lens 212. In some embodiments, the one or more CPB lenses may be magnetic lenses, and more specifically, objective lens 212 may be a magnetic objective (e.g., a magnetic immersion objective). In some embodiments, SEM 202 may be vertically positioned above sample S and may be used to image sample S. SEM 202 may include an electron source 214 and may be configured to manipulate the “raw” radiation beam from electron source 214 and perform operations such as focusing, aberration reduction, cropping (using aperture), filtering, etc. SEM 202 may generate an input beam 216 of charged particles (e.g., an electron beam) propagating along particle optical axis 218. SEM 202 may generally include one or more CPB lenses, such as condenser lens 210 and objective lens 212, to focus beam 216 onto sample S. In some embodiments, SEM 202 may be equipped with a deflection unit 220, which may be configured to manipulate beam 216. For example, the beam 216 can be manipulated across the sample S under study using scanning motion (e.g., grating or vector scanning). The objective lens 212 may include an excitation coil 222 surrounding a first pole 224 of the lens 212 to generate a focusing magnetic field in the gap between the first lens pole 224 and the sample S.

[0035] System 200 may further include a computer processing device and / or a control unit 250 for controlling, in particular, the deflection unit 220, the charged particle beam (CPB) lenses 210, 212 and the detector, and for displaying information collected from the detector on a display unit 252 (e.g., a computer, a mobile device, etc.).

[0036] As mentioned above, objective 212 is a magnetic immersion objective including excitation coil 222. However, the strength of the magnetic immersion lens 212 is limited by the maximum magnetomotive force generated by coil 222. Embodiments of this disclosure provide a solution to this problem by adding a magnetomotive force to lens 212 via a second lens pole and one or more permanent magnets that generate an additional magnetic field. Such embodiments can be used in STEM operations (e.g., high-resolution, high-voltage STEM) and / or in conventional SEM modes when using secondary or backscattered electrons.

[0037] System 200 may include a second movable stage 226, which includes a stage disposed adjacent to the substrate S (e.g., in...). Figure 2 The second lens pole 228 (also called the reverse pole 228) is shown in the orientation below the substrate. The first stage 206 can be, for example, a stage for transmission electron microscopy (TEM), and the second stage 226 can be a conventional SEM or a dual-bean stage (see example). Figure 5The reverse electrode 228 may include a central aperture 230 and a detector 232 disposed below the aperture 230. If the sample S is sufficiently thin (e.g., preferably less than 50 nm for semiconductor materials and less than 100 nm for biological materials), electrons can travel through the sample S and be detected by the detector 232. The angular distribution of the scattering angles of the detected electrons can provide information about the sample S. In some embodiments, the detector 232 may be, for example, a high-angle annular dark-field detector (HAADF detector), an annular dark-field detector (ADF detector), and / or a bright-field detector.

[0038] One or more magnets 234 may be coupled to the reverse pole to enhance the performance of the magnetic immersion objective 212 (e.g., improve resolution). The magnets 234 may be permanent magnets (e.g., permanent neodymium magnets or permanent samarium cobalt magnets). The permanent magnets 234 generate their own persistent magnetic field under a predetermined excitation. In some specific embodiments, adding a permanent magnet 234 to the reverse pole 228 is equivalent to adding approximately 1500 AT (ampere-turns) of excitation to the coil 222.

[0039] In other specific embodiments, the addition of permanent magnets can achieve higher beam energy by increasing the strength of the magnetic field. The magnet adds magnetic flux to the magnetic circuit, thereby increasing the strength of the magnetic field. A higher magnetic field allows for the use of higher-energy electron beams. Further details of the advantages of permanent magnets in electron microscopy can be found, for example, in Hawkes et al., *Principles of Electron Optics, Volume II: Applied Geometric Optics*. Principles of Electron Optics Volume Two: Applied Geometrical Optics The book can be found in Academic Press (2nd edition) (2017), which is incorporated into this article with full citation.

[0040] In other embodiments, each magnet 234 may be an electromagnet including a corresponding current-carrying coil.

[0041] In some specific embodiments, the sample S may be placed on the surface of the reverse pole 228. The magnet 234 may be laterally adjacent to the sample arrangement to enable magnetic immersion of the system without an immersion lens coil.

[0042] Magnet 234 may be disposed around optical axis 218. In some embodiments, magnet 234 may be disposed symmetrically about axis 218. In other embodiments, magnet 234 may be positioned asymmetrically about axis 218 to, for example, introduce desired asymmetry or compensate for undesired asymmetry in other components of the optical system. Magnet 234 may be disposed on one or more surfaces of antipole 228. For example, in the illustrated embodiment, magnet 234 is disposed on a first surface 236 adjacent to the antipole of sample S (e.g., on...). Figure 2The upper surface in the orientation shown in the diagram). In other embodiments, instead of or in addition to disposing the magnet 234 on the first surface 236, one or more magnets 234 may be disposed on the second surface 237 of the opposite pole (e.g., on the upper surface in the orientation shown in the diagram). Figure 2 On the lower surface of the orientation shown in the illustration. Although in the illustrated embodiment, the magnet 234 is disposed below the sample S (in Figure 2 In the orientation shown in the diagram, in other embodiments, the magnet 234 may be positioned laterally adjacent to the sample S. In yet another embodiment, the magnet 234 may be positioned... Figure 2 The orientation shown is placed above the sample.

[0043] Although the illustrated embodiments show only a single magnet 234 positioned at each circumferential location around the optical axis, in other embodiments, the magnets 234 may be stacked on top of each other, such that one or more magnets may be positioned at selected circumferential locations. In some such embodiments, the magnets may be joined together using, for example, an adhesive. In other embodiments, the magnets may have an interlocking shape.

[0044] Refer to the top view of the reverse pole 228. Figure 3 The surface 236 of the reverse electrode 228 adjacent to the sample S is provided (e.g., in...). Figure 2 The upper surface (as shown in the orientation) may include one or more grooves or notches 238 in which the magnet 234 may be disposed. In some embodiments, each notch 238 may be configured (e.g., by size and shape setting) to prevent unintentional movement of the magnet 234 relative to the reverse pole 228 (e.g., during movement of the reverse pole 228), such that it remains in the desired position and / or maintains a selected configuration relative to the optical axis 218. In some embodiments, each notch may have a shape corresponding to the shape of the corresponding magnet. For example, in the illustrated embodiment, each notch 238 has a circular shape in cross-section, the circular shape corresponding to the circular shape of the magnet 234.

[0045] In other embodiments, such as Figure 5 As shown, magnet 234 can be directly disposed on the first surface 236 of the reverse pole 228 and can be bonded to the surface using, for example, an adhesive.

[0046] Refer again Figure 3 In some embodiments, as shown, the notches 238 may be equidistantly spaced around the optical axis 218, forming a symmetrical pattern. In other embodiments, the notches 238 may be asymmetrically arranged around the optical axis 218. Magnets 234 may be symmetrically or asymmetrically arranged within the notches 238. For example, the asymmetrical arrangement of magnets 234 within the notches 238 around the optical axis 218 can be used to compensate for asymmetries in other components of the optical system. For example, in Figure 3 In the embodiment described herein, the reverse pole 228 includes twelve notches 238, three of which have magnets 234 disposed within the notches to form an asymmetrical pattern.

[0047] In some embodiments, such as Figure 4 As shown, the notch can be configured to extend radially from a position adjacent to the optical axis 218 to a track 240 at the edge 242 of the reverse pole 228. Each track 240 can be configured such that a corresponding magnet 234 can slide radially inward or outward relative to the optical axis 218 within the track 240. This configuration advantageously allows the magnet to be easily repositioned and / or adjusted relative to the axis 218 to compensate for any existing asymmetries in the system 200. In some embodiments, more than one magnet may be provided in each track 240. Although the illustrated embodiment shows multiple tracks 240 extending in the radial direction, in other embodiments, the tracks may extend circumferentially around the optical axis 218.

[0048] In some embodiments, as mentioned, the reverse pole 228 may be movable relative to the sample S. For example, the reverse pole 228 may be movable such that it can be completely removed from the sample chamber and / or stored in a "stage" connected to the sample chamber. In some embodiments, the reverse pole 228 may be moved from a first position within the vacuum chamber to a second position within the vacuum chamber, in which the reverse pole 228 is positioned sufficiently close to the magnetic immersion lens 212 to add additional magnetomotive force to the immersion lens (e.g., an "active" position), and in the second position, the reverse pole 228 does not affect the lens 212 (e.g., an "inactive" position). The reverse pole 228 may be excited by the immersion lens 212 (e.g., the reverse pole 228 may be part of the magnetic path of the immersion lens and does not need to be excited by a separate magnetic field, such as an auxiliary magnetic coil).

[0049] In other embodiments, the permanent magnet 234 may be disposed on the retractable optical element. Details of such configuration can be found in U.S. Patent Publication 2014 / 0110597, which is incorporated herein by reference in its entirety.

[0050] When system 200 is shown only as containing SEM columns, in some embodiments, system 200 may further include ion columns similar to ion column 104 of the previously described dual-beam system 100. In such embodiments, the magnetic immersion lens (including a counter pole 228 with a permanent magnet 234) described herein can be used to focus the ion beam from the ion column.

[0051] Example 3 Figure 6A representative graph illustrating the magnetic flux lines of the exemplary system 300 is provided. Region 302 corresponds to coil 222, the flux lines shown in region 304 correspond to the flux lines in the first lens pole 224, the flux lines shown in region 308 correspond to the flux lines in the second lens pole 228, and the flux lines shown in region 310 correspond to the flux lines in the vacuum chamber 204. As shown, region 302 illustrates the limited power of the existing immersion lens coil. The magnetic field strength within the first pole of the lens is shown in region 304. Region 306 illustrates the saturation of the magnetic circuit of the immersion lens. Such saturation, and the resulting magnetic field asymmetry, can adversely affect the performance of the lens. The embodiments described herein are configured to address these problems and advantageously allow the magnet to be used within the column itself, with a partially open magnetic circuit.

[0052] For example, Figure 7 This diagram illustrates representative flux lines of an exemplary system 400 containing permanent magnets. The flux lines in region 402 correspond to those in permanent magnet 234, region 404 to coil 222, the flux lines shown in region 406 to the first lens pole 224, the flux lines shown in region 408 to the second lens pole 228, and the flux lines shown in region 410 to the vacuum chamber 204. Region 402 illustrates the magnetic field strength added by permanent magnet 234, which can be equivalent to, for example, a coil of approximately 1500 AT.

[0053] Given the many possible embodiments to which the principles of this disclosure can be applied, it should be recognized that the illustrated embodiments are merely preferred examples and should not be considered as limiting the scope. In fact, the scope is defined by the appended claims. Therefore, we claim all subject matter falling within the scope and spirit of the appended claims. The alternatives specifically presented in the above examples are merely illustrative and do not constitute all possible alternatives to the embodiments described herein.

Claims

1. A charged particle beam system, comprising: Vacuum chamber; A sample holder for fixing the sample within the vacuum chamber; and A charged particle column includes a charged particle source for generating a charged particle beam along an optical axis and a magnetic immersion lens for focusing the charged particle beam, the magnetic immersion lens comprising: A first lens electrode is disposed adjacent to the first surface of the sample. An excitation coil that surrounds the first lens pole, and A reverse pole, configured adjacent to a second surface of the sample, comprising one or more magnets disposed on the surface of the reverse pole. The magnet is arranged asymmetrically around the optical axis.

2. The system of claim 1, wherein the surface of the reverse pole is a first surface, and wherein one or more additional magnets are disposed on the second surface of the reverse pole.

3. The system of claim 1, wherein the reverse pole comprises one or more notches, and wherein each magnet is disposed within a corresponding notch.

4. The system of claim 3, wherein the notch comprises a track, and wherein the one or more magnets are slidable within the track such that the magnets are positionable relative to the optical axis.

5. The system of claim 4, wherein each track extends radially inward from the outer edge of the reverse pole toward the optical axis.

6. The system of claim 4, wherein each track extends circumferentially around the optical axis.

7. The system of claim 1, wherein one or more magnets are coupled to the opposite pole via an adhesive.

8. The system of claim 1, wherein the reverse electrode is mounted to a positioning system configured to allow the reverse electrode to move between a first position and a second position within the vacuum chamber, wherein the reverse electrode is inactive in either the first or the second position.

9. The system of claim 1, wherein the reverse electrode further comprises an aperture and a detector, the detector being disposed within or below the aperture.

10. The system of claim 1, wherein the asymmetry of the magnet is configured to compensate for the magnetic circuit asymmetry of the system.

11. The system of claim 1, wherein the asymmetry of the magnet is configured such that it produces magnetic circuit asymmetry in the system.

12. A dual-beam system comprising: A sample holder, used to hold the sample in place; An ion beam column configured to guide an ion beam to the sample; and A charged particle column includes a charged particle source for generating a charged particle beam along an optical axis and a magnetic immersion lens for focusing the charged particle beam, the magnetic immersion lens comprising: A first lens electrode, disposed adjacent to a first surface of the sample, and A second lens pole, disposed adjacent to a second surface of the sample, includes one or more magnets. The magnet is arranged asymmetrically around the optical axis.

13. The system of claim 12, wherein one or more magnets are disposed on a first surface of the second lens pole, and wherein one or more additional magnets are disposed on a second surface of the second lens pole.

14. The system of claim 12, wherein the second lens pole includes one or more notches, and wherein each magnet is disposed within a corresponding notch.

15. The system of claim 12, wherein the dual-beam system further includes a vacuum chamber, and wherein the second lens pole is mounted to a positioning system configured to allow the second lens pole to move between a first position and a second position within the vacuum chamber, wherein the second lens pole is inactive in either the first or the second position.

16. The system of claim 12, wherein the second lens pole further includes an aperture and a detector disposed within or below the aperture.

17. The system of claim 12, wherein the magnetic immersion lens is coupled to the control unit.

18. A method of using the dual-beam system according to claim 12, comprising: The magnet is positioned around the optical axis to generate magnetic circuit asymmetry within the system.

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