Charged particle lens

Abstract

A charged particle lens for focusing a beam of charged particles towards a sample.SOLUTION: The charged particle lens comprises a first pole shoe 114, a second pole shoe 116 electrically insulated from the first pole shoe 114, a lens coil and at least one voltage supply, wherein a central axis 118 of the charged particle lens extends through central openings of the first pole shoe 114 and the second pole shoe 116, and wherein the lens coil is arranged to generate a magnetic field in the first pole shoe 114 and the second pole shoe 116; The at least one voltage supply is arranged to apply a potential difference between the second pole piece 116 and the sample 112 to generate an electric field. The generated magnetic field and the generated electric field focus a beam of charged particles passing through the central openings of the first pole piece 114 and the second pole piece 116 towards the sample 112.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a charged particle lens for focusing a beam of charged particles toward a sample mounted at a sample position. The charged particle lens can be used in a scanning electron microscope, and more particularly, can be disposed at the end of a booster tube of a scanning electron microscope to generate an electromagnetic immersion field. A scanning electron microscope including the charged particle lens and a method for focusing a beam of charged particles toward a sample mounted at a sample position are also described. [Background technology]

[0002] A scanning electron microscope (SEM) is a type of microscope that generates images by transmitting a beam of charged particles (such as electrons) onto the surface of a sample. The charged particles interact with the sample's surface, and information about the sample can be obtained based on detection of the charged particles reflected and / or emitted from the sample surface. SEM can provide information and imaging of the sample's surface topology, as well as information about the sample's composition. SEM imaging allows for the generation of particularly high-resolution images, in some cases with a resolution better than 1 nm.

[0003] Modern SEMs use booster tubes to accelerate electrons as they pass through the SEM column, thus minimizing the effects of environmental stray fields and charging effects, such as Coulomb interactions. The optical properties of an SEM with a booster tube (including its ability to focus, expand, and de-focus a charged particle beam) are primarily determined by the final optical element (e.g., a lens) located at the end of the booster tube; any charged particles must pass through this final optical element before impinging on the sample. The final optical element can be an objective lens for focusing the beam onto the sample. The objective lens can be, for example, a simple magnetic lens, an immersion lens in which the sample is immersed in a magnetic field, or more recently, compound lenses have been developed that incorporate both magnetic immersion and electrostatic lenses. Particularly when SEMs utilize low-energy electrons, compound lenses have been found to provide improved image resolution due to the accelerating electrostatic field parallel to the immersion magnetic field, which minimizes optical aberrations by providing the maximum electromagnetic field as close as possible to the sample.

[0004] Current state-of-the-art SEMs typically utilize booster tubes capable of delivering charged particles with low landing energies (e.g., with energies of approximately 50 eV). The booster tube is an optically continuous tube held at a constant high-voltage potential (e.g., +5 to +10 kV). The end of the booster tube necessarily forms an electrostatic lens. This electrostatic lens acts as a focusing lens on the electron beam, simultaneously varying the electron energy. The position of the electrostatic lens is important for the optical performance of the SEM. The general teaching in the art is that the closer the electrostatic lens is positioned to the focal point of the charged particle beam, the better the optical performance will be (e.g., the closer the electrostatic lens is to the sample, the better the optical performance will be).

[0005] The compound lens further comprises a magnetic lens. In prior art compound lenses, an electrostatic lens and a magnetic lens are arranged in series, each providing an effect on the charged particle beam. The final magnetic lens in an SEM generally has better quality than the electrostatic lens, and therefore it is typical to design the compound electrostatic-magnetic lens so that focusing due to magnetic refraction is the dominant effect. The final magnetic lens is a magnetic immersion lens, often referred to as a monopole magnetic lens. A schematic diagram of a known compound lens is shown in FIG. 1 , illustrating the focusing of a charged particle beam 10 onto a sample surface 12 by an electrostatic lens 14 and a magnetic lens 16. Again, it is recognized that the closer the magnetic lens is to the focal plane, the better the overall optical properties of the final lens. Examples of known compound lenses for demonstrating focusing as shown in FIG. 1 can be found in EP 2706554 and EP 2833390.

[0006] In view of the above, it would be desirable to provide a charged particle lens with improved performance compared to the prior art. Summary of the Invention

[0007] In a first aspect, a charged particle lens for focusing a beam of charged particles towards a sample mounted at a sample position is described, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and a sample mounted at the sample location to generate an electric field; The generated magnetic field and the generated electric field are for focusing a beam of charged particles passing through the central openings of the first and second pole pieces.

[0008] In a second aspect, a scanning electron microscope, SEM, with a charged particle lens is described.

[0009] In a third aspect, a method of focusing a beam of charged particles towards a sample mounted at a sample position is described, the method comprising: providing a charged particle lens, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and a sample mounted at the sample location to generate an electric field; The method comprises: The method further includes passing the beam of charged particles through the central openings of the first pole piece and the second pole piece toward the sample while generating the magnetic and electric fields, the generated magnetic and electric fields being for focusing the beam of charged particles toward the sample. [Brief explanation of the drawings]

[0010] The present disclosure can be put into practice in various ways, some of which will now be described, by way of example only, and with reference to the accompanying drawings. [Figure 1] 1 shows a schematic diagram of a prior art complex lens; [Figure 2] 1 shows a cross-sectional view of an example of a disclosed charged particle lens. [Figure 3] A cross-sectional view of a charged particle lens of an SEM is shown. [Figure 4] 4 shows a schematic diagram of the charged particle lens of FIG. 3. [Figure 5] 1 illustrates a plot of the magnetic field generated in a first exemplary configuration of a charged particle lens. [Figure 6] 10 illustrates a plot of the magnetic field generated in a second exemplary configuration of the charged particle lens. [Figure 7] 1 illustrates a cross section of a charged particle lens illustrating an example of an insulating element.

[0011] It will be understood that like features are labeled using like reference numerals.The drawings are not necessarily to scale. DETAILED DESCRIPTION OF THE INVENTION

[0012] Detailed explanation of specific examples This disclosure relates to a scanning electron microscope. This paper discusses a charged particle lens (which may also be considered a compound lens, or a charged particle compound electrostatic-magnetic immersion lens) that may be used in a scanning electron microscope (SEM). In particular, the described charged particle lens comprises a single element that provides both a magnetic lens and an electrostatic lens. Combining both the electrostatic and magnetic lenses of the compound lens within a single physical element removes the requirement for alignment of the focusing fields, thus reducing parasitic aberrations resulting from misalignment. The described charged particle lens may provide several other advantages, as described below.

[0013] FIG. 2 shows a cross-sectional view of an exemplary charged particle lens according to the present disclosure. FIG. 3 shows a schematic cross-sectional view of a charged particle lens in the column of an SEM. The charged particle lens comprises a first pole piece 114 and a second pole piece 116. A lens coil 122 (shown only in FIG. 3 ) is disposed around the outer surface of the first pole piece 114 such that a magnetic field is generated in the first pole piece 114 and the second pole piece 116 when the lens coil 122 is energized. The generated magnetic field is an immersion magnetic field and has a maximum magnitude located within the sample chamber and near the sample position where the sample 112 is mounted when the SEM is in use. The magnetic field acts as a magnetic lens to focus the charged particle beam passing through the charged particle lens toward the sample 112.

[0014] At least one voltage supply is arranged to apply a potential difference between the first pole piece 114 and the second pole piece 116, and between the second pole piece 116 and the sample 112 at the sample position. In the specific example of FIG. 2, the first voltage supply 130 is configured to apply a voltage to the second pole piece 116, which is electrically isolated from both the first pole piece 114 and the sample 112. In the example of FIG. 2, the sample position (or sample holder) is also connected to the second voltage supply 131, which can apply a voltage of positive or negative polarity (thereby also maintaining the sample 112 at a particular voltage). Appropriate selection of the voltages applied at the first pole piece and the second voltage supply generates a potential difference between the second pole piece and the sample 112 mounted at the sample position. The potential difference in turn generates an electric field that acts as an electrostatic lens to focus the charged particle beam passing through the charged particle lens towards the sample 112. As an alternative to the example of Figure 2, it will be appreciated that either the sample position or the second pole piece can be connected to earth ground, with a voltage applied to the other element via a voltage supply, to generate a potential difference between the sample 112 and the second pole piece 116 at the sample position.

[0015] The first pole piece 114 is an element having a generally frusto-conical shape on its outer surface and having a central opening or bore extending therethrough (resulting in a truncated tip of the cone). The pole piece is a structure constructed from a highly permeable material that functions to direct the magnetic field generated by the lens coil 122; the first pole piece 114 is formed from a material that is both ferromagnetic and electrically conductive.

[0016] The second pole piece 116 is a generally conical or cylindrical element with a central opening or bore extending therethrough. The second pole piece 116 is disposed to extend at least partially from the first pole piece 114 toward the sample location. In particular, the second pole piece 116 is disposed to be aligned with the first pole piece 114 such that the central axis 118 of the charged particle lens extends through the central openings of the first pole piece 114 and the second pole piece 116. Thus, a continuous passage extends through the central opening of the first pole piece 114 and then through the second pole piece 116.

[0017] The second pole piece 116 is formed from a material that is both ferromagnetic and electrically conductive. Thus, the high magnetic permeability of the material allows the second pole piece 116 to direct a magnetic field, while the electrical conductivity allows for the establishment of an electric field when a voltage is applied.

[0018] The second pole piece 116 is spaced apart and electrically isolated from the first pole piece 114. In some cases, electrical isolation may be provided solely by spacing or separating the first pole piece 114 and the second pole piece 116, but in the example of Figure 2, an insulating element 120 is disposed between the first pole piece 114 and the second pole piece 114. The insulating element 120 provides stable alignment between the first pole piece 114 and the second pole piece 116 and provides reliable electrical isolation between the two pole pieces, even in the presence of a large potential difference.

[0019] 3 shows the charged particle lens of FIG. 2 (comprising first pole piece 114, second pole piece 116, and insulating element 120) in a zoomed-out final lens assembly of an SEM. In particular, the charged particle lens is located at one end of a booster tube 124 of the SEM. The first pole piece 114 surrounds the booster tube 124 of the SEM, and the second pole piece 116 extends from the first pole piece 114 at one end of the booster tube 124. A central axis 118 extending through central openings of both the first pole piece 114 and the second pole piece 116 is also aligned with the central axis of the booster tube 124. The insulating element 120 acts to insulate the second pole piece 116 from the booster tube 124 and from the first pole piece 114.

[0020] A lens coil 122 is disposed around the outer surface of the first pole piece 114 and connected to the magnetic circuit. When energized, the lens coil 122 generates a magnetic field between the first pole piece 114 and the second pole piece 116 and the sample 112 mounted at the sample position. The magnetic field is an immersion magnetic field, with the magnetic field having a maximum strength in the vicinity of the sample 112 (and the sample position).

[0021] The SEM further comprises several charged particle detectors. In the example of FIG. 3 , the first detector 126 is a backscattered electron detector housed within the booster tube 124 at the end closest to the second pole piece 116. The backscattered electron detector is disposed inside the bore of the booster tube 124 and has an aperture therethrough. The booster tube also houses a second detector 128, a secondary electron detector, which is also disposed within the bore of the booster tube 124 but is disposed farther from the sample 112 than the backscattered electron detector. The secondary electron detector also has a central aperture aligned with the central axis 118. The apertures through the backscattered electron detector and the secondary electron detector allow the passage of a charged particle beam generated in a charged particle source (source not shown) and transmitted therefrom through the SEM.

[0022] Although not shown in FIGS. 2 or 3 , it will be understood that one or more controllers are connected to the SEM. The controllers may control (individually or together) one or more of the magnetic circuit, the motors for moving the sample holder and adjusting the sample position, at least one voltage supply 130, 131, and any charged particle detectors (including backscattered electron detectors and secondary electron detectors). The one or more controllers may form part of a computer-implemented control system, which may receive data from the detectors and other aspects of the system for further processing. The illustrated one or more controllers may control at least one voltage supply 130, including initiating and controlling the magnitude of a potential difference between the second pole piece 116 and each of the first pole piece 114 and / or the sample 112 mounted at the sample position. In one example, the controller may initiate and adjust the magnitude of a voltage applied to the second pole piece 116 to generate a required electric field.

[0023] When the SEM is in use, a charged particle beam passes from the source toward the sample 112 in the direction of the central axis 118, passing through the central apertures of the secondary and backscattered electron detectors and the central apertures of the first and second pole pieces 114, 116. Electrostatic and magnetic fields generated in the charged particle lens act to focus the charged particle beam onto the surface of the sample 112 at a focal plane. Charged particles incident on the sample 112 may then be reflected from the sample surface (and received in the backscattered electron detector 126) or may cause the emission of electrons from the sample surface (which may travel back through the booster tube and be detected in the secondary electron detector 128). Measurement of the charged particles in the charged particle detectors 126, 128 allows the SEM to form an image.

[0024] More specifically, when the charged particle lens is in use to focus a charged particle beam, a magnetic circuit (not shown) energizes lens coils 122, which are arranged to generate magnetic fields in the first and second pole pieces 114, 116. The first and second pole pieces 114, 116 are magnetized as a result of including ferromagnetic material. Simultaneously, a voltage is applied to the second pole piece 116, which generates a potential difference between the second pole piece 116 and the first pole piece 114, as well as between the second pole piece 116 and the sample 112, which is mounted in the same position. The presence of the potential difference generates an electrostatic field. The magnetic and electrostatic fields each produce a focusing effect on the charged particle beam passing through the central opening of the first and second pole pieces 114, 116 (and the booster tube 124). Importantly, the magnetic and electrostatic fields are generated by the same components, which are the first and second pole pieces 114, 116. This contrasts with prior art designs for charged particle lenses that use two separate components to generate the magnetic and electrostatic fields, a magnetic lens and an electrostatic lens, respectively.

[0025] The inventors have observed several advantages resulting from the integration of a magnetic lens and an electrostatic lens in the described manner. In particular, the proposed design avoids the problem experienced in the prior art of misalignment between the magnetic and electrostatic lenses. Misalignment can increase aberrations in prior art compound lenses and reduce the possible resolution of images in an SEM. Such misalignment is not present in the charged particle lens of FIG. 3 because the magnetic and electrostatic lenses are integrated and occur in the same element (the second pole piece 116). Any effect caused by slight misalignment between the first and second pole pieces is insignificantly small and can be ignored. The focusing effect of the compound electrostatic-magnetic immersion lens is primarily determined by the pole piece 116. Thus, the charged particle lens of the present disclosure provides the best possible concentricity between the magnetic and electrostatic elements (e.g., maximizing the lateral overlap between the magnetic and electrostatic fields), providing better resolution images in an SEM, and avoiding the requirement for an alignment step by the SEM user or manufacturer. In fact, the very tip of the second pole piece 116 (which provides the immersion magnetic field) is configured so that it can be "floated" at a high potential. As a result, the most important optical element for the electrostatic magnetic lens is integrated into one piece of soft ferromagnetic (or ferrimagnetic) material, which is the second pole piece 116. There is no possibility of mismatch between the electrostatic and magnetic fields, since they are formed by the same physical element.

[0026] A schematic diagram of the focusing of the charged particle lens of Figures 2 and 3 is shown in Figure 4. Here it can be seen that the effective electrostatic lens 14 and the effective magnetic lens 16 are one and the same, and therefore their respective fields effectively overlap to generate electromagnetic fields for focusing the charged particle beam 110 onto the sample 112 at the sample location. This can be compared with the similar diagram of Figure 1, which shows the two stages of focusing of the charged particle beam performed in a prior art compound lens having separate magnetic and electrostatic lenses.

[0027] The proposed charged particle lens demonstrated significant improvements over prior art designs. One advantage of the proposed charged particle lens stems from the integration of a magnetic lens and an electrostatic lens. This allows the composite lens to be positioned closer to the focal plane (at the sample surface) of the charged particle lens. The principal plane of the composite charged particle lens is then shifted closer to the sample 112, which reduces on-axis aberrations.

[0028] As discussed above, an immersion magnetic field is generated by energizing the lens coil 122, and a magnetic field is generated at the first pole piece 114 and the second pole piece 116. As mentioned, a separation or gap is disposed between the first pole piece 114 and the second pole piece 116 to provide electrical insulation between the two elements. The separation can be achieved by inserting an insulating element 120 (or spacer) made of an electrically insulating material between the two pole pieces 114, 116. The result of the gap between the pole pieces 114, 116 (and the resulting significantly reduced magnetic permeability at that point) is a discontinuity or break in the magnetic circuit, which causes a local peak or local maximum to occur in the magnetic field and align with the discontinuity. This local peak or maximum may be considered a parasitic magnetic lens, which itself may affect the beam of charged particles.

[0029] Preferably, the charged particle lens is configured to minimize the effect of the parasitic magnetic lens. In particular, this effect is minimized by designing the arrangement of the first pole piece 114 and the second pole piece 116 to maximize the overlap between (unintentional) peaks in the magnetic field caused by discontinuities in the magnetic circuit, which have smaller local maxima, and (intentional) peaks in the magnetic field that occur near the sample mounted at the sample position and have the magnitude of the global maximum in the magnetic field. The peaks with the magnitude of the global maximum provide the desired immersion magnetic field. Maximizing the overlap is achieved by configuring the second pole piece 116 and the first pole piece 114 so that the insulating gap or insulating element 120 (e.g., the discontinuity in the magnetic circuit) is as close as possible to the pole tip of the magnetic circuit. In other words, the gap (and insulating element 120) is disposed as close as possible to the sample 112 while still ensuring that the second pole piece 116 extends from the first pole piece 114 so as to be closer to the sample 112 than the first pole piece 114 (i.e., the portion of the second pole piece 116 between the first pole piece 114 and the sample provides the "tip" of the pole piece, e.g., the pole tip).

[0030] The effect of discontinuities in the magnetic circuit can be seen by comparing Figures 5 and 6. Figure 5 shows an example of a charged particle lens with the elements described above with respect to Figures 2 and 3, namely, first and second pole pieces 114 and 116 that are electrically isolated from one another. In the example of Figure 5, electrical isolation is provided by insulating element 120 that fills the gap 138 or separation between first and second pole pieces 114 and 116 and is disposed in close proximity to sample 112.

[0031] A plot of the magnetic field magnitude in the axial direction is shown in FIG. 5. It can be seen that a local maximum 132 exists in the magnetic field in the region of the gap 138, or the separation between the first and second pole pieces (in other words, the axial location of the discontinuity in the magnetic circuit). In the example of FIG. 5, the local maximum 132 caused by the gap 138 between the first and second pole pieces 114 and 116 is located at a distance z of approximately 12 mm from the surface of the sample (the sample is located at z=0 mm in the plot of FIG. 5). However, due to the proximity of the gap 138 to the sample and its location near the tip 136 of the second pole piece 116, the peak in the magnetic field with the local maximum 132 (and associated with the presence of the gap) nearly overlaps with the peak in the magnetic field with the global maximum 134 (representing the intentional immersion magnetic field). Therefore, the presence of the gap 138 between the pole pieces 114, 116 (and the resulting discontinuity in the magnetic circuit) does not significantly degrade the performance of the charged particle lens.

[0032] For comparison, FIG. 6 shows another example of a charged particle lens according to the elements described above with respect to FIGS. 2 and 3 (having a first pole piece 114 and a second pole piece 116 electrically isolated from each other by a separating gap 138). In the example of FIG. 6, the gap 138 disposed between the first pole piece 114 and the second pole piece 116 is located farther from the sample 112 than in the example of FIG. 5. In the example of FIG. 6, it can be seen that the local maximum caused by the gap 138 between the first pole piece 114 and the second pole piece 116 is located approximately 76 mm from the surface of the sample. Again, the magnitude of the axial magnetic field is shown, and it can be seen that a local maximum 132 exists in the magnetic field at the axial location of the gap 138. Note that although the example of FIG. 6 does not include insulating element 120, local maximum 132 still exists because it is caused by a gap in the magnetic circuit of first pole piece 114 and second pole piece 116.

[0033] In the case of Figure 6, the separation between the local maximum 132 and the global maximum 134 (which provides the immersion magnetic field) is such that the local maximum 132 is distinct and does not overlap with the global maximum 134. In the example configuration of Figure 6, the local maximum 132 of the magnetic field acts as a parasitic magnetic lens that has an effect separate from the immersion magnetic field. The parasitic magnetic lens reduces the overall performance of the charged particle lens. The performance of the proposed charged particle lens is substantially improved when the gap between the first and second pole pieces is positioned as close as possible to the sample.

[0034] In view of the above, in the preferred example of the charged particle lens described, the gap or spacing 138 between the first pole piece 114 and the second pole piece 116 is disposed as close as possible to the pole tip 136 of the charged particle lens. The separation between the gap or spacing 138 and the sample is then also minimized. Consequently, the gap or spacing 138 is disposed so that the local maximum in the magnetic field caused by the magnetic field discontinuity overlaps as closely as possible with the immersion magnetic field peak. In practice, this requires minimizing the axial length of the tip portion of the second pole piece 116 (the portion that protrudes from the first pole piece toward the sample), and ideally, the width of the gap or spacing 138 between the first pole piece 114 and the second pole piece 116 (and therefore the thickness of the insulating element 120).

[0035] The charged particle lens shown in FIGS. 2 and 3 includes an insulating element 120 having a "barbed" shape. The insulating element 120 is shown in more detail in FIG. 7. Specifically, the insulating element 120 is a ring arranged to have a "v" shape in radial cross section, with the insulating element 120 having an outermost surface forming a frustoconical shape. In other words, the insulating element 120 includes an open-ended cylindrical portion 140 having a bore extending in the direction of the central axis, and further includes wings 142 extending outward from one end of the open-ended cylindrical portion. In this manner, the pointed "tip" 144 of the wall of the first pole piece 114 is disposed in the valley of the "v" shape of the insulating element 120 between the cylindrical portion 140 and the wings 142. The insulating element 120 is disposed within a valley or rim 148 at the wider end of the substantially conical second pole piece 116, which forms the pole tip.

[0036] The shape of the insulating element 120 is optimized to provide electrical insulation between the first pole piece 114 and the second pole piece 116, even when a large potential difference is applied between the first and second pole pieces. In particular, the shape and material of the insulating element 120 must withstand large electrical gradients and creepage. In this manner, the shape of the insulating element 120 shown in FIG. 7 is advantageous because it maximizes the surface area between the high-voltage pole piece (typically the second pole piece 116) and the grounded pole piece (typically the first pole piece 114), while also satisfying other constraints, as described below. In particular, the shape of the insulating element 120 is based on the following criteria: · Maintain electrostatic field strength below 7000V / mm. · Maintaining a distance between the surfaces of two pole pieces at different potentials to provide a separation of 1 mm for every 1000 V potential difference. · Ensures that there is no line-of-sight contact between the insulating element 120 and the charged particle beam (the primary electron beam traveling towards the sample in the direction of the central axis 118). · Insulating element 120 ensures that there is no line-of-sight contact with charged particles (electrical signal beams) emitted or reflected from sample 112.

[0037] The design of insulating element 120 shown in Figure 7 is the result of optimizing these constraints. Exemplary dimensions (in mm), including the thickness of insulating element 120 required given the voltage intended to be applied to second pole piece 116 (which may be in the range of 50 to 5000 V), are shown in Figure 7. However, these dimensions should not be considered limiting.

[0038] Although embodiments according to the present disclosure are described with reference to particular types of devices and applications (particularly those used in scanning electron microscopes), and the embodiments have particular advantages in such cases, as discussed herein, the approaches according to the present disclosure may be applied to other types of devices and / or applications. Certain features may be omitted or substituted, for example, as indicated herein. Each feature disclosed herein, unless otherwise stated, may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless otherwise stated, each feature disclosed is merely an example of a generic series of equivalent or similar features.

[0039] In this detailed description of various embodiments, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. However, those skilled in the art will understand that these various examples and / or embodiments may be practiced with or without these specific details. Furthermore, those skilled in the art will readily understand that the specific order in which the methods are presented and performed is illustrative, and it is contemplated that the order may be changed and still remain within the scope of the various examples and / or embodiments disclosed herein.

[0040] As used herein, including within the claims, singular forms of terms are construed to include plural forms, and vice versa, unless the context indicates otherwise. For example, singular referents in this specification, including within the claims, such as "a" or "an," mean "one or more" unless the context indicates otherwise. Throughout the specification and claims of this disclosure, words such as "comprise," "including," "having," and "contain," as well as variations of words such as "comprising" and "comprises" or the like, mean "including but not limited to" and are not intended to exclude other elements. Additionally, the use of "or" is inclusive, so that a phrase such as "A or B" applies when "A" is true, when "B" is true, or when both "A" and "B" are true.

[0041] The use of any and all examples or exemplary language (such as "for instance," "such as," "for example," and similar language) provided herein is intended merely to better illustrate the invention and does not pose a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any element not claimed as essential to the practice of the disclosure.

[0042] The terms "first" and "second" may be reversed without changing the scope of the invention. That is, an element referred to as a "first" element or location may instead be referred to as a "second" element or location, and an element referred to as a "second" element or location may instead be considered a "first" element or location.

[0043] Any steps described herein may be performed in any order, or simultaneously, unless otherwise stated or required by context. Furthermore, if a step is described as being performed after another step, this does not exclude intervening steps from being performed.

[0044] Furthermore, unless otherwise implicitly or explicitly understood or stated, it will be understood that for any given component or embodiment described herein, any of the possible candidates or alternatives listed for that component may be generally used individually or in combination with one another. Furthermore, unless otherwise implicitly or explicitly understood or stated, it will be understood that any listing of such candidates or alternatives is merely illustrative and not limiting.

[0045] The following numbered clauses provide further illustrative examples only. 1. A charged particle lens for focusing a beam of charged particles toward a sample mounted at a sample position, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and a sample mounted at the sample location to generate an electric field; The charged particle lens, wherein the generated magnetic field and the generated electric field are for focusing a beam of charged particles passing through the central openings of the first pole piece and the second pole piece. 2. The charged particle lens of clause 1, wherein the magnetic field is an immersion magnetic field. 3. The charged particle lens of clause 1 or clause 2, wherein the first pole piece and / or the second pole piece are formed from a material that is ferromagnetic or ferrimagnetic and electrically conductive. 4. A charged particle lens according to any one of clauses 1 to 3, wherein the second pole piece is arranged to be spaced apart from the first pole piece by a gap in the direction of the central axis. 5. A charged particle lens as described in clause 4, wherein the first pole piece and the second pole piece are arranged so as to minimize the width of the gap in the direction of the central axis while maintaining electrical insulation between the first pole piece and the second pole piece. 6. A charged particle lens as described in clause 4 or clause 5, wherein the first pole piece and the second pole piece are arranged so that the overlap of a primary peak in the magnetic field, having a maximum value that is a global maximum in the magnetic field, with a secondary peak in the magnetic field caused by the gap is maximized. 7. A charged particle lens described in any one of clauses 4 to 6, wherein at least a tip portion of the second pole piece is arranged to extend closer to the sample position in the direction of the central axis than any portion of the first pole piece. 8. A charged particle lens as described in clause 7, wherein the tip portion has a non-zero depth, the depth being the distance between a surface of a first pole piece closest to the sample position and a second pole piece closest to the sample position, and the second pole piece is configured to minimize the non-zero depth of the tip portion. 9. The charged particle lens of any of clauses 1 to 8, further comprising an insulating element disposed between the first pole piece and the second pole piece for electrically insulating the first pole piece from the second pole piece. 10. The charged particle lens of any of clauses 1-9, wherein the charged particle lens is for use in a scanning electron microscope (SEM). 11. A scanning electron microscope (SEM) comprising a charged particle lens according to any one of clauses 1 to 10. 12. a booster tube extending at least partially through the central opening of the first pole piece in the direction of the central axis; 12. The SEM of clause 11, further comprising one or more charged particle detectors disposed in the booster tube for receiving charged particles emitted or reflected from the sample. 13. A method of focusing a beam of charged particles toward a sample mounted at a sample position, comprising: providing a charged particle lens, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and a sample mounted at the sample location to generate an electric field; The method comprises: The method further includes passing a beam of charged particles through the central openings of the first pole piece and the second pole piece toward the sample while generating a magnetic field and an electric field, the generated magnetic field and the generated electric field being for focusing the beam of charged particles toward the sample. 14. The method of clause 13, wherein the first pole piece and / or the second pole piece are formed from a material that is ferromagnetic or ferrimagnetic and electrically conductive.

[0046] In a first illustrated example, there is a charged particle lens for focusing a beam of charged particles towards a sample mounted at a sample position, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and a sample mounted at the sample location to generate an electric field; The generated magnetic field and the generated electric field are for focusing a beam of charged particles passing through the central openings of the first and second pole pieces.

[0047] The charged particle lens (or compound electrostatic-magnetic immersion lens) is for use in an SEM and may be disposed as the final element of a booster tube of the SEM. The charged particle lens may be used to focus (or magnify or demagnify) a beam of charged particles onto the surface of a sample disposed at a sample position (or sample face, or sample mount, or sample table). Primarily, charged particle lenses are used with electrons, but it will be understood that they may be used to direct other ions.

[0048] The first and second pole pieces are elements for directing the magnetic field lines. In particular, the first and second pole pieces direct the magnetic field generated by the lens coil so that the magnetic field has a maximum magnitude near the sample position (or specifically near the sample at the sample position). In this way, the first and second pole pieces provide an immersion magnetic field in which the sample is immersed when the charged particle lens is in use.

[0049] The first pole piece and the second pole piece each have a central opening (or central bore) therethrough. The central openings (or bores) of the first pole piece and the second pole piece are aligned so that a central axis can pass directly through the openings of both pole pieces. This alignment is such that a charged particle beam can pass directly through the central openings of the first pole piece and the second pole piece without deflection or redirection. In use, the charged particle beam passes substantially along the central axis. The sample position (and in use, the sample) is aligned with the central axis so that the charged particle beam passing through the central openings of the first pole piece and the second pole piece is incident on the surface of the sample at the sample position.

[0050] The first and second pole pieces are electrically isolated from each other. This allows a voltage to be applied to either the first or second pole piece, generating a potential difference between the two pole pieces. In particular, one or more voltage supplies are arranged to apply a potential difference between the second pole piece and the sample position or holder (and the sample thereon). This can be done by applying a voltage to the second pole piece via the first voltage supply and / or by connecting the first pole piece and / or the sample position to ground or a second or additional voltage supply. Because the second pole piece is electrically isolated from the first pole piece and the sample position, the second pole piece can be held at a higher voltage than both the first pole piece and the sample position (and the sample). By applying this potential difference, the electrically isolated second pole piece, held at a higher voltage relative to its adjacent elements, generates an electrostatic field between it and the sample position (and the first pole piece).

[0051] The electrostatic field generated by applying a voltage to the insulated second pole piece and the magnetic field generated by the lens coil and directed by the first and second pole pieces cause focusing of the charged particle beam as it passes through the central openings of the first and second pole pieces (i.e., along the central axis) and toward the sample. The electrostatic field can be considered to provide an electrostatic lens, and the magnetic field can be considered to provide a magnetic lens. The electrostatic and magnetic fields together provide an electromagnetic field that produces the focusing effect. In this case, focusing the charged particle beam can include expanding or contracting the beam (in other words, changing the beam width at the focal plane). The second pole piece is crucial for generating the electrostatic and rotationally symmetric magnetic fields responsible for focusing the charged particles. The integration of the components of the charged particle lens that generate the electrostatic and magnetic fields provides advantages, as described elsewhere in this disclosure.

[0052] As discussed above, the generated magnetic field is an immersion magnetic field. Therefore, any sample placed or mounted at the sample position is immersed in the magnetic field. The immersion magnetic field has a maximum magnitude (which is the global maximum of the magnetic field) near the sample position. The peak global maximum can be in the sample chamber downstream of the second pole piece. Providing an immersion magnetic field has been shown to reduce aberrations when used in SEM booster tubes.

[0053] The second pole piece is formed from a ferromagnetic or ferrimagnetic and electrically conductive material. The first pole piece is also formed from a ferromagnetic or ferrimagnetic and electrically conductive material. This allows the pole pieces to be used to direct the generated magnetic field and to support the application of a potential difference between the second pole piece and other adjacent elements (such as the first pole piece and the sample). Ferromagnetic materials have observable magnetic permeability and can typically form permanent magnets. Ferrimagnetic materials are materials with populations of atoms that have opposing but unequal magnetic moments, thus remaining spontaneously magnetized. Examples of suitable ferromagnetic and electrically conductive materials for forming the first and second pole pieces include, but are not limited to, soft ferromagnetic materials such as pure iron, extra-low carbon steel, nickel-based alloys (commonly known as Permalloy), or cobalt-based alloys (commonly known as Permendur or Hyperco). The first pole piece and the second pole piece may be made of different types of materials that are ferromagnetic (or ferrimagnetic) and electrically conductive.

[0054] Preferably, the second pole piece is arranged to be spaced apart from the first pole piece by a gap in the direction of the central axis. In other words, a portion of the second pole piece (specifically, the tip portion) extends closer to the sample than the first pole piece in the direction of the central axis. In some cases, the second pole piece is arranged such that its first end is concentric with the first pole piece and its second end extends away from the first pole piece toward the sample position. The first pole piece may be substantially frustoconical, and the second pole piece provides a conical tip.

[0055] The second pole piece is electrically isolated from the first pole piece. This can be achieved by physically separating, displacing, or spacing the second pole piece from the first pole piece. The separation or gap between the first and second pole pieces ensures electrical isolation. However, the gap or separation between the first and second pole pieces typically results in a local maximum in the generated magnetic field aligned with the gap in the direction of the central axis. This is due to a discontinuity in the magnetic circuit. This local maximum in the generated magnetic field has a parasitic magnetic lensing effect. Beneficially, the gap or separation between the first and second pole pieces can be located as close as possible to the sample position in the direction of the central axis to move the local maximum in the generated magnetic field toward the sample position. Ideally, the local maximum in the generated magnetic field is located near the sample position and coincides with or overlaps as closely as possible with the magnitude of the global peak or global maximum of the magnetic field that provides the immersion magnetic field discussed above.

[0056] In other words, the first and second pole pieces may be arranged to maximize the overlap of a primary peak of the magnetic field, having a maximum value that is the global maximum of the magnetic field, with a secondary peak of the magnetic field caused by the gap, the secondary peak having a maximum value that is smaller, typically much smaller, than the global maximum. In particular, the first and second pole pieces are arranged to avoid an excessively large secondary peak in the magnetic field as a result of the presence and location of the gap.

[0057] The length of the second pole piece extending from the surface of the first pole piece closest to the sample position toward the central axis can be minimized to shift any peaks with local maxima in the magnetic field so that they overlap as closely as possible with peaks associated with the immersion magnetic field, while still generating an electric field. Considered another way, at least a tip portion of the second pole piece can be arranged to extend closer to the sample position in the direction of the central axis than any portion of the first pole piece. The tip portion can have a non-zero depth, the depth being the distance between the surface of the first pole piece closest to the sample position and the surface of the second pole piece closest to the sample position, and the second pole piece can be configured to minimize the non-zero depth of the tip portion. In other words, at least a portion of the second pole piece (the "tip portion") protrudes from the first pole piece toward the sample position, and the depth of the tip portion is finite and non-zero, but is minimized as much as possible. In this way, the gap between the first pole piece and the second pole piece is located as close as possible to the tip of the charged particle lens closest to the sample mounted at the sample position.

[0058] Additionally or alternatively, the first and second pole pieces may be arranged to minimize the width of the gap (or separation) between the first and second pole pieces in the direction of the central axis while maintaining electrical isolation between the first and second pole pieces. Minimizing the size of any gap or separation minimizes the width of any peaks that describe local maxima in the magnetic field.

[0059] In one example, the distance between the sample position and the center of the gap in the direction of the central axis may be less than four times the distance between the sample position and the surface of the second pole piece closest to the sample position in the direction of the central axis. In another example, the distance between the sample position and the center of the gap in the direction of the central axis may be less than two times the distance between the sample position and the surface of the second pole piece closest to the sample position in the direction of the central axis.

[0060] Preferably, the charged particle lens further comprises an insulating element disposed between the first pole piece and the second pole piece for electrically insulating the first pole piece from the second pole piece. The insulating element may be a spacer or a washer made from an electrically insulating material. The insulating element may be an electrical insulating element. Examples of materials that may be included in or used to form the insulating element include any non-conductive (i.e., insulating) material that a) can be machined with high precision, b) has a relative permittivity as close to 1 as possible, and c) is compatible with use in high vacuum. Possible materials for forming the insulating material include machinable glass ceramics (Macor, etc.). RTM , polyether ether ketone (PEEK), alumina, or aluminum nitride.

[0061] The insulating element may fill a gap or separation between the first pole piece and the second pole piece. The insulating element may provide mechanical support for holding the second pole piece relative to the first pole piece. The insulating element may have a central opening, and the insulating element may be disposed between the first pole piece and the second pole piece, such that a central axis of the charged particle lens extends through the first pole piece, the insulating element, and the central opening of the second pole piece. In other words, at least a portion of the second pole piece may be disposed between the insulating element and the sample in the direction of the central axis.

[0062] In one example, the insulating element may include an open-ended cylindrical portion having a central bore extending therethrough in the direction of the central axis and may further include wings extending outward from the ends of the open-ended cylindrical portion. In other words, the outermost surface of the insulating element may be frustoconical with an internally disposed cylindrical portion, the cylinder having a bore or opening therethrough. This may give the insulating element the appearance of an open-ended, barbed cylinder. The surfaces of the first and second pole pieces adjacent to the insulating element may be configured to conform to the shape of the insulating element, such that the surface of the first pole piece is positioned to seat within the valley of the "v" portion between the barb and the cylinder. The described shape of the insulating element may increase the breakdown voltage of the electrical insulation between the first and second pole pieces by increasing the surface area of ​​the insulating element disposed between the first and second pole pieces.

[0063] Charged particle lenses can be used in scanning electron microscopes (SEMs).

[0064] A second example is a scanning electron microscope (SEM) equipped with the above-described charged particle lens. The SEM may further include a booster tube extending at least partially through the central opening of the first pole piece and / or the second pole piece in the direction of the central axis. The SEM may further include one or more charged particle detectors disposed within the booster tube to receive charged particles emitted or reflected from the sample. In particular, the first charged particle detector may be a backscattered electron detector disposed within the central opening of the first pole piece. The second charged particle detector may be a secondary electron detector aligned with the central axis such that the first pole piece is disposed between the second pole piece and the secondary electron detector. The charged particle lens may be particularly useful for use in an SEM. This is because the implementation of a single element (a second pole piece that is a combination magnetic and electrostatic lens) for directing and focusing the electrostatic and magnetic fields reduces image aberrations in the SEM compared to prior art designs that have separate electrostatic and magnetic lenses.

[0065] In a third example, there is provided a method of focusing a beam of charged particles toward a sample mounted at a sample position, the method comprising: providing a charged particle lens, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the central openings of the first and second pole pieces, the central openings of the first and second pole pieces being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field in the first pole piece and the second pole piece; providing a charged particle lens comprising at least one voltage supply arranged to apply a potential difference between a second pole piece and a sample mounted at the sample location to generate an electric field; The method comprises: The method further includes passing a beam of charged particles through the central openings of the first pole piece and the second pole piece toward the sample while generating a magnetic field and an electric field, the generated magnetic field and the generated electric field being for focusing the beam of charged particles toward the sample.

[0066] Preferably, the first pole piece and / or the second pole piece are made from a material that is ferromagnetic or ferrimagnetic and electrically conductive.

[0067] Preferably, the second pole piece is spaced apart from the first pole piece by a gap in the direction of the central axis. The first and second pole pieces may be arranged to maximize the overlap between a primary peak of the magnetic field, having a maximum value that is the global maximum of the magnetic field, and a secondary peak of the magnetic field caused by the gap. At least a tip portion of the second pole piece may be arranged to extend closer to the sample position in the direction of the central axis than any portion of the first pole piece. The tip portion may have a non-zero depth, the depth being the distance between the surface of the first pole piece closest to the sample position and the surface of the second pole piece closest to the sample position, and the second pole piece may be configured to minimize the non-zero depth of the tip portion. Additionally or alternatively, the size of the gap (in the direction of the central axis) may be minimized while maintaining electrical insulation between the first and second pole pieces.

[0068] The method may further include providing an insulating element disposed between the first pole piece and the second pole piece to electrically isolate the first pole piece from the second pole piece, The insulating element may fill a gap between the first pole piece and the second pole piece.

[0069] In a fourth example, there may be a method for obtaining an image of a sample in a scanning electron microscope (SEM) by using the described charged particle lens.

Claims

1. a charged particle lens for focusing a beam of charged particles towards a sample mounted at a sample position, said charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field at the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and the sample mounted at the sample location to generate an electric field; a charged particle lens, wherein the generated magnetic field and the generated electric field are for focusing a beam of charged particles passing through the central openings of the first pole piece and the second pole piece.

2. The charged particle lens of claim 1 , wherein the magnetic field is an immersion magnetic field.

3. The charged particle lens of claim 1 , wherein the first pole piece and / or the second pole piece are formed from a material that is ferromagnetic or ferrimagnetic and electrically conductive.

4. The charged particle lens according to claim 1 , wherein the second pole piece is disposed so as to be spaced apart from the first pole piece by a gap in the direction of the central axis.

5. 5. The charged particle lens of claim 4, wherein the first pole piece and the second pole piece are arranged such that the width of the gap in the direction of the central axis is minimized while maintaining electrical insulation between the first pole piece and the second pole piece.

6. 5. The charged particle lens of claim 4, wherein the first pole piece and the second pole piece are arranged to maximize overlap of a primary peak in the magnetic field, having a maximum that is a global maximum in the magnetic field, with a secondary peak in the magnetic field caused by the gap.

7. 5. The charged particle lens of claim 4, wherein at least a tip portion of the second pole piece is arranged to extend closer to the sample position in the direction of the central axis than any portion of the first pole piece.

8. 8. The charged particle lens of claim 7, wherein the tip portion has a non-zero depth, the depth being the distance between a surface of the first pole piece closest to the sample position and the second pole piece closest to the sample position, and the second pole piece is configured to minimize the non-zero depth of the tip portion.

9. 10. The charged particle lens of claim 1, further comprising an insulating element disposed between the first pole piece and the second pole piece for electrically insulating the first pole piece from the second pole piece.

10. The charged particle lens of claim 1 , wherein the charged particle lens is for use in a scanning electron microscope (SEM).

11. A scanning electron microscope (SEM) comprising the charged particle lens of claim 1.

12. a booster tube extending at least partially through the central opening of the first pole piece in the direction of the central axis; 12. The SEM of claim 11, further comprising one or more charged particle detectors disposed in the booster tube for receiving charged particles emitted or reflected from the sample.

13. 1. A method for focusing a beam of charged particles toward a sample mounted at a sample location, comprising: providing a charged particle lens, the charged particle lens comprising: a first pole piece having a central opening; a second pole piece electrically insulated from the first pole piece and having a central opening, the second pole piece being disposed to be aligned with the first pole piece so that a central axis of the charged particle lens extends through the first pole piece and the central openings of the second pole piece, the central openings of the first pole piece and the second pole piece being for passing the beam of charged particles toward the sample; a lens coil arranged to generate a magnetic field at the first pole piece and the second pole piece; at least one voltage supply arranged to apply a potential difference between the second pole piece and the sample mounted at the sample location to generate an electric field; The method comprises: the method further comprising passing the beam of charged particles through the central openings of the first pole piece and the second pole piece toward the sample while generating the magnetic field and the electric field, wherein the generated magnetic field and the generated electric field are for focusing the beam of charged particles toward the sample.

14. The method of claim 13 , wherein the first pole piece and / or the second pole piece are formed from a material that is ferromagnetic or ferrimagnetic and electrically conductive.

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