Magnetic field-free sample plane for charged particle microscopy

Abstract

To provide an apparatus for changing a magnetic field in a sample plane of an electron microscope, such as a TEM or an STEM.SOLUTION: A charged particle microscope at least includes first and second optical elements arranged on opposing sides of a sample plane, a third optical element arranged around the sample plane, and a controller coupled to control the first, second and third optical elements. The controller is coupled to excite the first and second optical elements to generate first and second magnetic lenses, the first and second magnetic lenses being formed on opposing sides of the sample plane and being oriented in the same direction, and excites the third optical element to generate a third magnetic lens at the sample plane that is oriented in an opposite direction. A ratio of the excitation of the third optical element to the excitation of the first and second optical elements adjusts a magnetic field at the sample plane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates generally to charged particle microscope lenses, and more particularly to charged particle microscope objective lenses that produce negligible or zero magnetic fields at the sample plane. [Background technology]

[0002] Magnetic lenses are used in charged particle microscopy as objective lenses when the charged particles are highly energetic. The objective lenses are used to focus and image the charged particle beam. Such objective lenses use a strong magnetic field near or around the sample, resulting in a short focal length. While this is not an issue for most materials, imaging with such lens configurations can damage or destroy sensitive magnetic materials. Traditional approaches may involve moving the magnetic lens further away from the sample to reduce or eliminate the magnetic field at the sample. However, such approaches result in an increase in the focal length, which reduces the resolution of the charged particle microscope. While such techniques for imaging magnetic materials have achieved some success, the loss of resolution remains a problem, and further improvements are desired. Summary of the Invention

[0003] Disclosed herein is an exemplary apparatus for varying a magnetic field at a sample plane of an electron microscope, such as a TEM or STEM. The exemplary apparatus includes at least first and second optical elements, the first and second optical elements arranged around the sample plane. The first and second optical elements can be arranged above and below the sample plane, with the above and below being along the optical axis of the electron microscope. The microscope further includes a third optical element also arranged around the sample plane. The electron microscope further includes a controller coupled to control the first, second, and third optical elements to affect the magnetic field at the sample plane. The controller may be coupled to or include a non-transitory memory storing code that, when executed by the controller, causes the controller to excite the first and second optical elements to create first and second magnetic lenses formed around the sample plane, the first and second lenses being oriented in the same direction, and to excite the third optical element to create a third magnetic lens at the sample plane, the third magnetic lens being oriented in the opposite direction to the first and second magnetic lenses, where a ratio of the excitation of the first and second optical elements to the excitation of the third optical element adjusts the magnetic field at the sample plane. The magnetic field modulation results from the interaction of the third magnetic lens at the sample plane and the magnetic fields present at the sample plane from the first and second magnetic lenses.

[0004] Disclosed herein is an exemplary method for varying a magnetic field at a sample plane of an electron microscope, the exemplary method including at least: forming first and second magnetic lenses around the sample plane based on excitation of first and second coils, the first and second magnetic fields being oriented in the same direction and symmetrically positioned on opposite sides of the sample plane; forming a third magnetic lens at the sample plane based on excitation of a third coil, the third magnetic field being oriented in a direction opposite to the orientations of the first and second magnetic lenses; and adjusting the magnetic field at the sample plane based on a ratio of the excitation of the third coil to the excitation of the first and second coils. [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is an exemplary charged particle microscope according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is an exemplary objective lens according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is an exemplary method for adjusting a magnetic field at a sample plane of a charged particle microscope according to an embodiment of the present disclosure. [Figure 4A] FIG. 4A is an exemplary magnetic field plot according to an embodiment of the present disclosure. [Figure 4B] FIG. 4B is an exemplary magnetic field plot according to an embodiment of the present disclosure. [Figure 5] FIG. 5 is an exemplary functional block diagram according to one embodiment of the present disclosure.

[0006] Like reference numerals refer to corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0007] Embodiments of the present invention are described below in the context of a charged particle microscope including one or more objective lenses configured to form an adjustable magnetic field at the sample plane. For example, three lenses can be arranged around the sample plane and energized to cancel or generate a small magnetic field at the sample plane. In some embodiments, the field-free objective lens is energized symmetrically or nearly symmetrically, while the immersion objective lens is energized on the opposite side of the field-free objective lens. In such embodiments, excitation of the immersion objective lens reduces or cancels the magnetic field generated at the sample plane due to the field-free objective lens. However, it should be understood that the methods described herein are generally applicable to a wide range of different tomography methods and devices, including both cone-beam and parallel-beam systems, and are not limited to a particular device type, beam type, object type, length scale, or scan trajectory.

[0008] As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "includes" means "comprises." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between the coupled items.

[0009] The systems, devices, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed to all novel and unobvious features and aspects of the various disclosed embodiments, alone and in various combinations and subcombinations with one another. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, nor do the disclosed systems, methods, and devices require that any one or more particular advantages be present or problems be solved. While any theory of operation is for ease of explanation, the disclosed systems, methods, and devices are not limited to such theory of operation.

[0010] Although some operations of the disclosed methods are described in a particular order for convenience of presentation, it should be understood that this description style encompasses reordering, unless a specific order is required by specific terminology described below. For example, operations described sequentially can, in some cases, be reordered or performed simultaneously. Moreover, for simplicity, the accompanying figures may not show the various ways in which the disclosed systems, methods, and apparatuses can be used with other systems, methods, and apparatuses. Additionally, the description sometimes uses terms such as "generate" and "provide" to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and will be readily discernible to those skilled in the art.

[0011] In some instances, values, procedures, or devices are referred to as "lowest," "best," "smallest," etc. Such descriptions are intended to indicate that a selection from among many functional options may be made, and it will be understood that no such selection is necessarily better, lesser, or otherwise desirable than other options.

[0012] In charged particle microscopes, charged particles are focused and imaged by so-called "charged particle lenses." These lenses can be created by appropriately shaping magnetic or electric fields. However, for electron microscopes operating at beam potentials above 60 kV, typical of transmission electron microscopes (TEM) or scanning transmission electron microscopes (STEM), lenses made with electric fields are impractical due to the large electric fields required. For these types of microscopes, the preferred method for creating charged particle lenses is to use magnetic fields.

[0013] Traditionally, magnetic lenses are constructed as follows: a coil carrying hundreds of wire windings is arranged rotationally symmetrically around the optical axis of the microscope. The coil is excited with a current of several amperes. At such high currents, the electrical resistance of the wire in the coil can lead to significant power losses. This is typically addressed by water cooling the coil and by using large-diameter wire. Such coils have a diameter of at least approximately 10 centimeters and a cross-section (perpendicular to the current) of at least approximately 10 square centimeters. While excited, the coil generates a strong magnetic field axially (i.e., parallel to the optical axis) on the optical axis of the microscope. Because the magnetic field has no divergence, it has an inherently radial (i.e., perpendicular to the optical axis) component. These radial components grow proportionally to the distance to the optical axis, and their net effect is a deflection of charged particles toward the optical axis, scaling proportionally to the distance of these particles to the optical axis. Thus, such a magnetic field functions as a circular focusing lens.

[0014] The focusing strength of a magnetic lens is proportional to the square of the axial magnetic field and the length of the axial magnetic field (measured along the optical axis). Therefore, to obtain a strong magnetic lens, it is common to concentrate the magnetic field in a small area on the optical axis by using a magnetic yoke around a coil to induce magnetic flux and focus it in a small area on the axis. The length of such a small area is only a few millimeters. Although the magnetic lens is composed of the entire structure of the yoke, coil, and water cooling, the term "magnetic lens" as used herein refers only to the small area near the axis where the axial magnetic field is concentrated. Furthermore, as used herein, the term "optical element" refers to the structure (i.e., coil, yoke) that generates the magnetic field that forms the magnetic lens.

[0015] Powerful lenses are desirable because they provide the highest magnification and the least aberrations. Here, aberration refers to deviations from the ideal linear focusing effect. One of the most important aberrations is spherical aberration, which causes an additional focusing effect proportional to the cube of the distance from the particle to the optical axis. The spherical aberration of a magnetic lens is approximately linearly proportional to the length of the magnetic field (measured along the optical axis) and the cube of the focal length of the magnetic lens.

[0016] The resolution of a charged particle microscope is determined by the optical quality of the objective lens. In imaging microscopes such as TEM, the objective lens is the first imaging lens, while in scanning microscopes such as SEM and STEM, it is the final probe-forming lens. Therefore, to obtain the highest resolution, it is desirable to place the objective lens as close as possible to the specimen (to minimize the focal length) and to keep the length of the magnetic field as short as possible. For this purpose, modern charged particle microscopes often employ so-called immersion objectives, in which the specimen is inside the field of the objective lens. Such immersion ensures the shortest focal length and the shortest field length.

[0017] Immersion of specimens in the magnetic field of an objective lens is possible for a wide range of specimens. However, immersion is not possible for some types of specimens because they contain delicate magnetic structures (e.g., magnetic particles or magnetic vortices) that are altered or destroyed in the presence of an external magnetic field. Such specimens can only be studied under "field-free" conditions, i.e., using an objective lens that is not immersion-type and does not generate a magnetic field on the specimen. The leading approach to creating a field-free condition is to move the objective lens far enough away from the specimen so that the tail of its magnetic field is sufficiently reduced to zero so as not to affect any delicate magnetic structures within the specimen. However, this simple approach typically requires a very large distance between the objective lens and the specimen (and correspondingly large focal length and large aberrations), resulting in up to a tenth-fold decrease in resolution compared to immersion lenses, and atomic resolution is no longer achievable.

[0018] For some specimens with magnetic structures, it is desirable to image the specimen not only in an (essentially) field-free state, but also in the presence of a small, tunable magnetic field, where such a field can be varied to study the response of the magnetic structures as a function of the applied external field.

[0019] One approach to creating an objective lens relatively close to the specimen and with minimal magnetic tail at the specimen is to simultaneously create two symmetrically positioned, energized, field-free lenses on opposite sides of the sample plane and a (small) immersion objective lens at the sample plane. The two field-free lenses are energized equally so that their magnetic fields point in the same direction, while the immersion objective lens is energized so that its magnetic field points in the opposite direction. In this way, any magnetic field leakage from either field-free lens to the sample plane interacts with the field of the immersion objective lens such that the resulting magnetic field is reduced or eliminated. The ratio of the excitation of the immersion lens to the excitation of the field-free lens determines the state of the resulting magnetic field, e.g., zero or reduced to a small field (such a small, adjustable field is advantageous, for example, for studying the response of magnetic samples to external magnetic fields). The reduction or elimination of the magnetic field at the sample plane can only be achieved along the optical axis, i.e., the z-component of the resulting magnetic field; the x- and y-components (or radial components) may decrease a short distance from the optical axis before increasing. The excitation magnitude generated by an immersion objective lens can be smaller than the excitation magnitude generated by a field-free lens. As mentioned above, this relative operating configuration allows for the generation of a small magnetic field at the sample plane, or even zero magnetic field at the sample plane. Therefore, the magnetic field of the sample plane can be adjusted by changing the excitation ratio.

[0020] FIG. 1 illustrates an example of a charged particle microscope 100 according to an embodiment of the present disclosure. The charged particle microscope (CPM) 100 may, in some examples, be a TEM or a scanning TEM (STEM), although such designation is not limiting or necessary. The CPM 100 may include an objective lens positioned and configured so that the magnetic field at the sample plane can be adjusted. For example, the objective lens may be excited so that no magnetic field, at least in the direction of the optical axis, is generated at the sample plane, or only a small magnetic field is generated at the sample plane. Note that a radial component of the magnetic field may be present, but may also decrease with radial distance from the optical axis. By varying the level of the magnetic field present at the sample plane, magnetic materials can be imaged in various magnetic states. Alternatively, the objective lens may be excited to produce more conventional lens characteristics, such as a strong magnetic field generated at the sample plane, forming a conventional immersion lens. CPM 100 includes at least a source 102, a condenser system 104, an objective lens 106, a sample holder 108, a projection system 110, a detector 112, and a controller 114. The various components 102-112 may be coupled to controller 114 to control their functional aspects, such as lens excitation, projection control, and detector selection / data processing. It should be noted that CPM 100 may have additional or fewer components, and the components included in CPM 100 are for illustrative purposes only and are not limiting of this disclosure.

[0021] The source 102 can be any type of charged particle emitter, such as Schottky, field emission, or thermionic. A controller 114 issues control signals to one or more power supplies (not shown) coupled to the source 102 to generate a charged particle beam 116 of a desired current and energy. In some embodiments, the source 102 generates and emits an electron beam at an energy and current controlled by the controller 114.

[0022] The condenser system 104 includes various optical and beam steering components positioned and configured to direct and / or focus the beam 116 onto a sample in the sample holder 108. The controller 114 determines the shape and focus of the beam 116 at different planes within the CPM 100 and provides control signals that control the movement of the beam 116 across the sample on the sample holder 108.

[0023] The sample holder 108 may be an elongated rod that can move in and out of the optical axis of the CPM, as shown by beam 116 in FIG. 1 , and is configured to hold the sample such that electrons striking the sample continue along the optical axis for eventual detection by detector 112. The position of the sample holder 108 is located at the sample plane, as referenced herein. Furthermore, the sample holder 108 can be positioned with multiple degrees of freedom, for example, by a positioning device (not shown) that moves the sample in an XY plane perpendicular to the optical axis. Such movement allows various portions of the specimen S to be illuminated / imaged / inspected by the electron beam 116 moving along the optical axis (in the Z direction) (and / or allows for scanning motion, as an alternative to electron beam scanning). If desired, an optional cooling device (not depicted) can be in thermal contact with the specimen holder H, thereby maintaining the specimen holder H (and the specimen S thereon) at, for example, a cryogenic temperature.

[0024] The projection system 110 may include various electrostatic / magnetic lenses, deflectors, correctors (such as stigmators), etc. that are controlled to focus the beam 116 that traverses the sample onto the detector 112 .

[0025] The detector 112 can comprise several different detector modules that can be moved independently in the optical path to acquire transmitted charged particles that have passed through the sample. The number of different detectors can include at least a phosphor screen that can be imaged with a CCD, a solid-state sensor configured to directly detect charged particles, such as electrons, and / or a segmented detector that can map the location of impinging charged particles. Of course, other detectors are within the scope of this disclosure.

[0026] The controller 114 is coupled to control and receive data from various other components of the CPM 100. The controller 114 can provide various functions, such as synchronizing operations, providing set points, processing signals, performing calculations, and displaying messages / information on a display device (not depicted). Additionally, the controller 114 may include one or more processing cores, various types of memory, and one or more communication interfaces. In some examples, the controller 114 may be connected to cloud-based processing systems and data banks via any type of network, such as the Internet, a local area network, or a wide area network. The memory, whether included in the controller 114 or coupled to the controller 114 via a network, may include executable code that, when executed, controls the functions of the CPM 100 in accordance with the techniques discussed herein.

[0027] The objective lens 106 is arranged around the sample holder 108 and includes multiple optical elements. As used herein, the term "optical element" refers to a structure that generates a magnetic field on the optical axis. Conventionally, an optical element includes one or more coils surrounded by a magnetic yoke shaped to direct the magnetic flux generated by the surrounded coils to a small region of the optical axis when energized. Each of the optical elements generates a respective magnetic lens along an optical path inside the objective lens 106. The multiple magnetic lenses generated include at least two field-free objective lenses and an immersion objective lens. The two field-free objective lenses are formed around the sample plane. As used herein, around the sample plane means that the two magnetic lenses are formed above and below the sample plane in a direction parallel to the optical axis of the CPM 100, as represented by the electron beam 116, or, in other words, are positioned on opposite sides of the sample plane 108. The two field-free lenses can be positioned symmetrically around the sample plane 108 or approximately symmetrically around the sample plane 108. As used herein, "symmetrically disposed around the sample surface" means that each of the two magnetic field-free lenses is formed equidistant from the sample surface but on opposite sides of the sample surface, whereas "approximately symmetrically disposed around the sample surface" means that the distances from the sample surface to each magnetic field-free lens are not equal, so that one may be closer to or farther from the sample surface than the other. The immersion objective lens is formed at the sample surface and is generated in a volume that includes the sample surface.

[0028] Two magnetic-field-free objective lenses are formed from respective optical elements, each including a coil around which a yoke is wound and a pole piece from which the yoke extends toward the sample surface 108. The coil, yoke, and pole piece are positioned on opposite sides of the sample surface 108, with the pole piece extending toward the sample surface 108. An immersion objective lens can be formed from one or more coils positioned laterally adjacent to one or both of the coils / yokes of the two optical elements. The coils of the objective lenses may be different sizes or all the same size, with size referring to the number of windings. As understood in the art, the excitation of the coils is typically referred to in terms of ampere-turns (AT), which determines the strength and orientation of the lens formed thereby.

[0029] During operation, the controller 114 excites the objective lens 106 in various ways via one or more power sources (not shown) to adjust the magnetic field at the sample plane. For example, the objective lens 106 can be excited so that there is no magnetic field at the sample plane, at least along the optical axis, e.g., the z-component of the magnetic field. Alternatively, the objective lens 106 can be excited so that a small magnetic field is generated at the sample plane, again in the direction of the optical axis. Neutralizing or generating a small electric field enables, for example, imaging and analysis of magnetic materials. Additionally, the objective lens 106 can be operated in a more conventional manner, resulting in a strong immersion lens being formed at the sample plane. Such flexibility in providing field-free, small-field, and conventional imaging can be attributed to the coil design and symmetry of various components within the objective lens 106.

[0030] In some embodiments, the coils forming the field-free objective lens are excited symmetrically in the same direction, e.g., with the same sign of bias, while one or more coils forming the immersion objective lens are excited oppositely to the field-free objective coils. The excitation of the coils forming the field-free objective lens and the coils forming the immersion objective lens results in either zeroing the z-component of the magnetic field at the sample plane or generating a small magnetic field in the z direction. More specifically, the ratio of the two excitations can determine whether the magnetic field at the sample plane 108 is zeroed or reduced. To further explain, the magnetic fields from the two field-free lenses leak and spread to the sample plane 108, while the immersion objective lens can combine with the leaked magnetic field to zero or reduce the magnetic field at the sample plane. This combined magnetic field has a strength that depends on the ratio of the excitations. Note that the physical placement of the two sets of lenses relative to the sample plane also affects the magnetic field at the sample plane. Furthermore, it should be noted that the coils of a field-free objective need not be equally excited; instead, one coil may be excited at a fraction or multiple of the other. Such fractions or multiples may range, for example, from 0.5 to 2.

[0031] FIG. 2 illustrates an exemplary objective lens 206 according to an embodiment of the present disclosure. This diagram shows a cross section of the objective lens 206. The objective lens 206 is an example of the objective lens 106 and may be included in a charged particle microscope, such as a TEM or STEM, to name a few. The objective lens 206 can be positioned around a sample plane 208, which represents the location within the CPM where the sample is placed for image acquisition. The objective lens 206 can operate in a conventional mode, e.g., an immersion mode, where a large magnetic field is present at the sample plane 208, or the objective lens 206 can operate in a magnetic-field-free mode, e.g., where little or no magnetic field is present at the sample plane. To study magnetic materials, it is useful to be able to zero the magnetic field or generate a small magnetic field at the sample plane as needed.

[0032] The objective lens 206 includes at least three optical elements. A first optical element 220 is positioned on one side of the sample plane 208, and a second optical element 222 is positioned on the opposite side of the sample plane 208. The optical plane sides are referenced to an optical axis 216, which may be, for example, the optical axis for propagation of the electron beam. The third optical element includes coils 244 and 246, as well as a holding element 250 and portions of the yokes 240 and 242. This example shows an embodiment in which the third optical element includes two coils (244 and 246). The advantage of such separation of the two coils is that it allows easy access to the optical axis of the sample holder 208. Alternatively, the third element may include only one coil (244 or 246). The holding element 250 can function as a magnetic field applicator under certain operating conditions. A third optical element is also positioned around the sample surface 208, e.g., on either side of the sample surface, and adjacent to the first and second optical elements, respectively. Furthermore, the first and second optical elements 220, 222 are positioned such that a gap G is formed between their respective pole pieces, the gap being formed around the sample surface 208. While FIG. 2 shows a cross section of the objective lens 206, the various components may be formed in a toroidal shape centered about the optical axis 216.

[0033] The first optical element 220 includes a coil 228 and a yoke 240. The yoke 240 is shaped to have first and second pole pieces 230 and 232 formed therein and extend toward the sample surface 208. The two pole pieces 230 and 232 are separated by a gap at each end. The second optical element 222 includes a coil 234 and a yoke 242. The yoke 242 is shaped to have first and second pole pieces 236 and 238 formed therein and extend toward the sample surface 208. Similarly, the two pole pieces 236 and 238 are separated by a gap at each end. A gap G is formed from the distance between the pole piece 230 of the first optical element 220 and the pole piece 236 of the second optical element 222, and the gap G surrounds the sample surface 208.

[0034] The third optical element is formed by coils 244 and 246, the portion of yoke 240 including at least pole piece 230, retaining element 250, and the portion of yoke 242 including pole piece 236. Alternatively, the third optical element may have only one coil (such as coil 244 or coil 246). The third optical element may be positioned laterally adjacent to first and second optical elements 220 and 222, respectively, but generally at a substantially equal distance from opposite sample face 208.

[0035] The first and second coils 228 and 234 may be similarly configured with respect to windings. The third and fourth coils 244 and 246 may likewise be similarly configured with respect to one another. In some embodiments, all four coils 228, 234, 244, and 246 are similar in size. However, in other embodiments, the first and second coils 228, 234 may be larger or smaller than the third and fourth coils 244, 246.

[0036] During operation, the objective lens 206 can be energized to control the charged particle beam traveling along the optical axis 216 and to further manage / adjust / alter / modify the magnetic field at the sample plane 208. The energization of the first and second optical elements 220, 222 and / or the third and fourth coils 244, 246 can form multiple magnetic lenses within the objective lens 206. For example, the energization of the first optical element 220 forms a first magnetic lens at the ends of the pole pieces 230 and 232. Similarly, the energization of the second optical element 222 forms a second magnetic lens at the ends of the pole pieces 236 and 238. The first and second magnetic lenses are also sometimes referred to as field-free lenses, as is known in the art. The third optical element can also be energized to form an immersion objective lens at the sample plane 208. The immersion objective lens can be created by energizing one or both of the coils 244 and 246.

[0037] For example, in some embodiments, the first and second optical elements 220, 222 are symmetrically excited, meaning that they are excited with the same energy and generate a magnetic field in the same orientation, while the third and / or fourth coils 244, 246, i.e., the third optical element, are symmetrically excited relative to one another, but their resulting magnetic fields are in the opposite orientation to the magnetic fields generated by the first and second optical elements 220, 222. For example, the first and second optical elements 220, 222 may be excited with 4000 ampere-turns (AT), while the third and / or fourth coils 244, 246 are excited with -18 AT. In other embodiments, the excitation of the first and second optical elements is not symmetric with respect to energy, e.g., bias magnitude; instead, one of the first or second optical elements may be excited with a magnitude that is a fraction or multiple of the excitation provided to the other optical element. The fraction / multiple is proportional to the magnitude of the excitation, but the sign of the excitation will be the same. The fraction / multiple can range, for example, from 0.5 to 2. Of course, other excitations are possible, and the selected excitation can be based on the desired magnetic field conditions at the sample plane.

[0038] In the above excitation regime, the magnetic fields of both the first and second magnetic lenses can leak toward the sample plane 208. Because the two lenses are oriented in the same direction, this magnetic field leakage combines additively. However, because the immersion objective lens has an opposite orientation, the immersion objective lens-generated magnetic field also combines with the leakage magnetic field at the sample plane, and the resulting magnetic field is small or null. Whether the magnetic field at the sample plane is canceled or reduced depends on the relative excitation of the first and second optical elements relative to the third optical element. More specifically, the ratio of excitation of the third optical element to the excitation of the first and second optical elements adjusts the magnetic field at the sample plane. For example, with the geometry of Figure 2 and an electron acceleration voltage of 300 kV, the first and second optical elements can be excited to 4000 AT to create a magnetic lens that focuses 300 keV energy electrons onto the sample plane, and the third optical element can be excited to -18 AT to null the z-component of the magnetic field at the sample plane 208. Similarly, for the geometry of FIG. 2 , with an electron acceleration voltage of 100 kV, the excitation of the first and second optical elements can be 2100 AT, creating a lens that focuses 100 keV electrons onto the sample plane. The excitation of the third optical element can be set to −9.5 AT to zero the z-component of the magnetic field at the sample plane 208. For these two examples of acceleration voltages, the ratio of the excitation of the third optical element to the first and second optical elements is constant: −18:4000 = −9.5:2100 = −0.45%. Those skilled in the art will appreciate that the constancy of this ratio is a result of the linear relationship between coil excitation and magnetic field, that this linearity can be compromised by magnetic saturation, which can occur with high excitation and / or narrow magnetic yokes, and that this ratio is somewhat dependent on the excitation in the presence of magnetic saturation.

[0039] A 4000 AT excitation of the first and second optical elements is effective for the geometry shown in Figure 2 and electrons with a beam energy of 300 keV. In other embodiments, the excitation of the first and second optical elements required to focus 300 keV electrons can vary between 2000 AT and 8000 AT depending on the selected gap between pole pieces 230 and 240, the gap between 236 and 242, the inner diameter of the pole pieces, and the possible magnetic saturation within the pole pieces. Similarly, the ratio of the excitation of the third optical element to the excitation of the first and second optical elements required to null the magnetic field at the sample can vary between -0.01% and -5% depending on the selected geometry.

[0040] Note that the symmetry or near-symmetry of the excitation of the optical elements to reduce or eliminate the magnetic field at the sample plane 208 may not be the only factor affecting the resulting magnetic field. The physical symmetry of the objective lens 206 also affects the tuning of the magnetic field. For example, the symmetry of the pole pieces 230, 232 and 236, 238, along with the immersion lens formed at the sample plane, affects how the magnetic fields generated by the first and second optical elements interact. In such scenarios where the sample plane is not centered within the gap G, the relative excitation of the first and second optical elements can be adjusted to accommodate an offset sample plane 208. For example, optical elements further from the sample plane 208 can receive stronger excitation than other optical elements. Of course, an opposite excitation regime, e.g., closer optical elements receiving lower excitation, can also be implemented.

[0041] 3 is an exemplary method 301 for adjusting the magnetic field at the sample plane of a charged particle microscope, according to an embodiment of the present disclosure. Method 301 can be implemented in a charged particle microscope, such as CPM 100, for example. Method 301 can be used to study magnetic materials that are typically problematic to study with conventional CPMs, which typically have strong magnetic fields at the sample plane. By implementing method 301, such materials can be studied in the presence of zero or a small magnetic field.

[0042] Method 301 can begin at process block 303 with forming first and second magnetic lenses around the sample surface. The first and second magnetic lenses can be formed by first and second optical elements, such as optical elements 220 and 222. For example, first and second coils associated with the first and second optical elements can be energized to form first and second magnetic lenses, which can be formed on opposite sides of the sample surface. In some embodiments, the energization of the first and second coils can be symmetric with respect to at least the sign of the energization. Symmetric energization can result in magnetic lenses being generated in the same direction, at least relative to the optical axis of the CPM being performed. In some embodiments, the energization can also be symmetric with respect to the magnitude of the energization, as well as the sign of the coil current. However, in other embodiments, one magnetic lens can be formed from an optical element energized at a fraction or multiple of the energization of the other optical element, where the multiple is related to the magnitude of the energization. The multiple can range, for example, from 0.5 to 2. In such embodiments, the bias direction is the same, but the magnitude of the bias can differ between the two energizations.

[0043] Process block 303 may be followed by process block 305, which includes forming a third magnetic lens at the sample plane. The third magnetic lens may be oriented in a direction opposite to the orientation of the first and second magnetic lenses. The third magnetic lens may be generated by energizing a third optical element, i.e., a third and / or fourth coil. If both the third and fourth coils are energized, they may be energized symmetrically. Coils 244 and 246 are examples of third and fourth coils. Note that the magnitudes of excitation of the first and second optical elements and the third optical element may be different as well. For example, the magnitude of excitation of the first and second optical elements may be greater than the magnitude of excitation of the third optical element. In some embodiments, the first and second optical elements are excited at 4000 AT, and the third optical element is excited at -18 AT.

[0044] Process block 305 may be followed by process block 307, which includes adjusting the magnetic field at the sample plane based on a ratio of the excitation of the third coil to the excitation of the first and second coils. The ratio of excitations adjusts the amount of magnetic field in the axial direction, e.g., the z-direction or the optical axis. The excitation may be selected for a given sample or a given image acquisition to generate a desired magnetic field at the sample plane, such as zero or a small magnetic field. The small magnetic fields referred to herein are typically in the range of about 0.001 Tesla to about 0.1 Tesla, but sometimes as small as 0.0001 T to 0.001 T, and sometimes in the range of 0.1 T to 0.5 T.

[0045] 4A and 4B show exemplary magnetic field plots 401 and 403, respectively, according to one embodiment of the present disclosure. The data shown in plots 401 and 403 show the strength of the magnetic field relative to a sample plane, such as sample planes 108 and 208, in the Z and X directions, respectively, and an objective lens, such as objective lenses 106 and 206, excited as disclosed herein. For example, the first and second optical elements may be excited at 4000 AT, while the third optical element is excited at -18 AT, which minimizes the magnetic field at the sample plane. Plot 401 shows the variation of the z-component of the magnetic field at various positions on the optical axis around the sample plane. In plot 401, the sample plane is at position zero (0) on the x-axis of the plot, and as shown in the plot, the magnetic field at this position is very small. Based on the plot (y-axis), the magnetic field at z=0 is 3.0×10 -5 is less than T. Varying the excitation of the main lens, the auxiliary lens, or both, can cause the magnetic field at z=0 to change to zero.

[0046] Plot 403 shows the magnetic field in the x-direction (perpendicular to the optical axis), which includes representations of the x- and z-components of the magnetic field in the x-direction. As shown, the x-component of the magnetic field is zero at z=0 and is barely present at z=-1 mm. Furthermore, the z-component of the magnetic field increases with increasing x-distance until it reaches a maximum, after which it slowly decreases. Such data indicates that the objective lens may need to be de-energized when a magnetic sample is loaded and unloaded into a charged particle microscope.

[0047] 5 is an exemplary functional block diagram 500 according to one embodiment of the present disclosure. FIG. 5 is a block diagram illustrating a computer system 500 that may be used to implement one embodiment of the present disclosure. The computing system 500 may be one example of the computing hardware included in the system 100. The computer system 500 may include at least a hardware processor, such as a core 530, for processing information and may be coupled to a communication bus. The computing system 500 may be used to implement the methods and techniques disclosed herein, such as method 301, and may be used to acquire images based on synchronization of the pulse period of a pulsed electron beam with the scanning of a sample.

[0048] Computer system 500 also includes a main memory 532, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus for storing information and instructions executed by core(s) 530. Main memory 532 may also be used to store temporary variables or other intermediate information during execution of instructions executed by core(s) 530. Such instructions, when stored in a non-transitory storage medium accessible to core(s) 530, render computer system 500 into a special-purpose machine customized to perform the operations specified in the instructions.

[0049] Computer system 500 further includes a read-only memory (ROM) 534 or other static storage device coupled to the bus for storing static information and instructions for core 530. A storage device 536, such as a magnetic disk or optical disk, is provided and coupled to the bus for storing information and instructions.

[0050] Computer system 500 may be coupled via a bus to a display, such as display 31, for displaying information to a computer user. Input device 33, including alphanumeric and other keys, is coupled to the bus for communicating information and command selections to core 530. Another type of user input device is a cursor control, such as a mouse, trackball, or cursor direction keys, for communicating directional information and command selections to core 530 and for controlling cursor movement on the display. This input device typically has two degrees of freedom in two axes, i.e., a first axis (e.g., "x") and a second axis (e.g., "y"), that allow the device to specify a position in a plane.

[0051] Computer system 500 may implement the techniques described herein using customized hardwired logic, one or more ASICs or FPGAs, firmware and / or program logic that combines with a computer system to make computer system 500 a special-purpose machine, or to program computer system 500. According to one embodiment, the techniques described herein are performed by computer system 500 in response to core 530 executing one or more sequences of one or more instructions contained in main memory 532. Such instructions may be read into main memory 532 from another storage medium, such as storage device 536. Execution of the sequences of instructions contained in main memory 532 causes core 530 to perform the process steps described herein. In alternative embodiments, hardwired circuitry may be used in place of or in combination with software instructions.

[0052] The term "storage media," as used herein, refers to any non-transitory medium that stores data and / or instructions that cause a machine to operate in a specific manner. Such storage media may include non-volatile media and / or volatile media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 536. Volatile media include dynamic memory, such as main memory 532. Common forms of storage media include, for example, floppy disks, flexible disks, hard disks, solid-state drives, magnetic tape or other magnetic data storage media, CD-ROMs, any other optical data storage media, any physical media with a pattern of holes, RAM, PROM, EPROM, FLASH-EPROM, NVRAM, any other memory chip or cartridge, content addressable memory (CAM), and ternary content addressable memory (TCAM).

[0053] Storage media are distinct from but may be used in combination with transmission media. Transmission media involves transferring information between storage media. For example, transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 640. Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.

[0054] Various forms of media may be involved in carrying one or more sequences of one or more instructions to core 530 for execution. For example, the instructions may initially be carried on a magnetic disk or solid-state drive of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a network.

[0055] Computer system 500 also includes a communication interface 538 coupled to the bus. The communication interface 538 provides a two-way data communication coupling to a network link (not shown) connected to a local network. For example, communication interface 538 may be an Integrated Services Digital Network (ISDN) card, a cable modem, a satellite modem, or a modem providing a data communication connection to a corresponding type of telephone line. As another example, communication interface 538 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In such implementations, communication interface 538 sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0056] Computer system 500 can send messages and receive data, including program code, through the network(s), network link, and communication interface 538. In an Internet example, a server may transmit a requested code for an application program over the Internet through an ISP, a local network, and / or communication interface 538.

[0057] The received code may be executed by core 530 as it is received, and / or stored in storage device 536, or other non-volatile storage, for later execution.

[0058] The embodiments discussed herein to illustrate the disclosed technology should not be considered limiting but merely provide examples of implementations. Those skilled in the art will recognize myriad other ways in which the disclosed technology may be implemented that are contemplated herein and within the scope of this disclosure.

[0059] An exemplary apparatus for implementing the disclosed techniques, such as modifying a magnetic field at a sample plane of an electron microscope, such as a TEM or STEM, includes at least first and second optical elements, the first and second optical elements arranged around the sample plane. The first and second optical elements can be arranged above and below the sample plane, with the above and below being along the optical axis of the electron microscope. The microscope further includes a third optical element also arranged around the sample plane. The electron microscope further includes a controller coupled to control the first, second, and third optical elements to affect the magnetic field at the sample plane. The controller may be coupled to or include a non-transitory memory storing code that, when executed by the controller, causes the controller to excite the first and second optical elements to create first and second magnetic lenses formed around the sample plane, the first and second lenses being oriented in the same direction, and to excite the third optical element to create a third magnetic lens at the sample plane, the third magnetic lens being oriented in the opposite direction to the first and second magnetic lenses, where a ratio of the excitation of the first and second optical elements to the excitation of the third optical element adjusts the magnetic field at the sample plane. The magnetic field modulation results from the interaction of the third magnetic lens at the sample plane and the magnetic fields present at the sample plane from the first and second magnetic lenses.

[0060] In the exemplary apparatus described above, the ratio of excitation of the first and second optical elements to the excitation of the third optical element eliminates the axial component of the magnetic field at the sample plane.

[0061] In any of the above exemplary devices, the excitation of the first and second optical elements is between 2000 and 8000 ampere-turns, and the excitation of the third optical element is between -1 and -400 ampere-turns.

[0062] In any of the exemplary devices described above, the ratio of excitation of the first and second optical elements to excitation of the third optical element creates an immersion lens at the sample plane.

[0063] In any of the above exemplary devices, the excitation of the first and second optical elements is less than 2000 and the excitation of the third optical element is between 2000 and 8000 ampere-turns.

[0064] In any of the above exemplary devices, the magnitude of the second excitation is a multiple of the magnitude of the first excitation.

[0065] In any of the above exemplary devices, the multiple is in the range of 0.5 to 2.

[0066] In any of the above exemplary apparatus, the first and second optical elements include first and second coils, respectively, and first and second yokes, respectively, each including a pole piece extending from an opposite side toward the sample surface.

[0067] In any of the above exemplary devices, the third optical element includes a third and a fourth coil, the third coil being positioned adjacent to the first optical element and the fourth coil being positioned adjacent to the second optical element.

[0068] In any of the exemplary apparatuses described above, exciting the third optical element includes exciting only the third coil.

[0069] In any of the above exemplary apparatus, exciting the third optical element includes exciting the third and fourth coils with the same excitation.

[0070] An exemplary method for implementing the disclosed techniques, such as modifying a magnetic field at a sample plane of an electron microscope, such as a TEM or STEM, includes forming first and second magnetic lenses around the sample plane based on excitation of at least first and second coils, where the first and second magnetic fields are oriented in the same direction and are symmetrically positioned on opposite sides of the sample plane; forming a third magnetic lens at the sample plane based on excitation of a third coil, where the third magnetic field is oriented in a direction opposite to the orientation of the first and second magnetic lenses; and adjusting the magnetic field at the sample plane based on a ratio of excitation of the third coil to excitation of the first and second coils to excitation of at least the third coil.

[0071] In any of the above exemplary methods, the ratio of the excitation of the first and second coils to the excitation of the at least third coil eliminates an axial component of the magnetic field at the sample plane.

[0072] In any of the above exemplary methods, the excitation of the first and second coils is between 2000 and 8000 ampere-turns, and the excitation of the third optical element is between 1 and 400 ampere-turns.

[0073] In any of the above exemplary methods, the ratio of the excitation of the first and second coils to the excitation of the at least third coil creates an immersion lens at the sample plane.

[0074] In any of the above exemplary methods, the excitation of the first and second coils is less than 2000 and the excitation of the third optical element is between 2000 and 8000 ampere-turns.

[0075] In any of the above exemplary methods, the ratio of the excitation of the first and second coils to the excitation of the at least third coil produces a small magnetic field at the sample plane.

[0076] In any of the above exemplary methods, the excitation of the first and second coils is between 2000 and 8000 ampere-turns, and the excitation of at least the third coil is between -1 and -400 ampere-turns.

[0077] In any of the above exemplary methods, the magnitude of excitation of at least a third coil is a multiple of the magnitude of excitation of the first and second coils.

[0078] In any of the above exemplary devices, the multiple is in the range of 0.5 to 2.

[0079] In any of the exemplary methods above, a fourth coil is energized when forming a third magnetic lens at the sample plane, the fourth coil being energized in the same manner as the third coil.

Claims

1. 1. An apparatus comprising: a first optical element and a second optical element positioned on opposite sides of the sample plane; a third optical element disposed around the sample surface; a controller coupled to control the first optical element, the second optical element, and the third optical element, the controller, when executed by the controller, causing the controller to: exciting the first optical element and the second optical element to generate a first magnetic lens and a second magnetic lens, the first magnetic lens and the second magnetic lens being formed on opposite sides of the sample surface, and the first magnetic lens and the second magnetic lens being oriented such that magnetic fields of the first magnetic lens and the second magnetic lens point in the same direction; exciting the third optical element to generate a third magnetic lens, forming the third magnetic lens at the sample plane, the third magnetic lens oriented such that a magnetic field thereof faces in an opposite direction to the magnetic fields of the first magnetic lens and the second magnetic lens; a controller coupled to or including a non-transitory memory storing code for causing the Including, wherein a ratio of the excitation of the third optical element to the excitation of the first optical element and the second optical element adjusts the magnetic field at the sample plane.

2. The apparatus of claim 1 , wherein the ratio of the excitation of the third optical element to the excitation of the first optical element and the second optical element cancels an axial component of the magnetic field at the sample plane.

3. 3. The apparatus of claim 2, wherein the excitation of the first optical element and the second optical element is between 2000 and 8000 ampere-turns, and the excitation of the third optical element is between −1 and −400 ampere-turns.

4. 2. The apparatus of claim 1, wherein the ratio of the excitation of the third optical element to the excitation of the first optical element and the second optical element produces a small magnetic field at the sample plane, the small magnetic field having a magnitude in the range of 0.001 T to 0.1 T, in the range of 0.0001 T to 0.001 T, or in the range of 0.1 T to 0.5 T.

5. 5. The apparatus of claim 4, wherein the excitation of the first optical element and the second optical element is between 2000 and 8000 ampere-turns, and the excitation of the third optical element is between −1 and −400 ampere-turns.

6. The apparatus of claim 1 , wherein the ratio of the excitation of the first optical element and the second optical element to the excitation of the third optical element creates an immersion lens at the sample plane.

7. The device described in claim 1, wherein one of the first optical element and the second optical element is excited at a magnitude different from the magnitude of the excitation of the other optical element.

8. The device described in claim 7, wherein one optical element is excited at a magnitude that is 0.5 to 2 times the magnitude of the excitation of the other optical element.

9. The first optical element and the second optical element are a first coil and a second coil, respectively; each including a first yoke and a second yoke; 2. The apparatus of claim 1, wherein the first yoke and the second yoke each include a pole piece extending toward the sample surface.

10. 2. The apparatus of claim 1, wherein the third optical element includes a third coil and a fourth coil, the third coil being positioned adjacent to the first optical element and the fourth coil being positioned adjacent to the second optical element.

11. The apparatus of claim 10 , wherein the excitation of the third optical element comprises exciting only the third coil.

12. The apparatus of claim 10 , wherein the excitation of the third optical element includes exciting the third coil and the fourth coil with the same excitation.

13. generating a first magnetic field and a second magnetic field based on excitation of a first coil and a second coil, and forming a first magnetic lens and a second magnetic lens around a sample surface by the first magnetic field and the second magnetic field, wherein the first magnetic field and the second magnetic field are oriented in the same direction and are symmetrically disposed on opposite sides of the sample surface; generating a third magnetic field based on excitation of a third coil, and forming a third magnetic lens at the sample plane with the third magnetic field, the third magnetic field being oriented in a direction opposite to the orientations of the magnetic fields of the first magnetic lens and the second magnetic lens; and adjusting a magnetic field at the sample plane based on a ratio of the excitation of the third coil to the excitation of the first coil and the second coil.

14. The method described in claim 13, wherein the ratio of the excitation of the third coil to the excitation of the first coil and the second coil eliminates the axial component of the magnetic field at the sample surface.

15. 15. The method of claim 14, wherein the excitation of the first coil and the second coil is between 2000 and 8000 ampere-turns, and the excitation of the third coil is between −1 and −400 ampere-turns.

16. The method of claim 13 , wherein a ratio of the excitation of the first coil and the second coil to the excitation of the third coil creates an immersion lens at the sample plane.

17. 14. The method of claim 13, wherein the ratio of the excitation of the first coil and the second coil to the excitation of the third coil produces a small magnetic field at the sample plane, the small magnetic field having a magnitude in a range of 0.001 T to 0.1 T, in a range of 0.0001 T to 0.001 T, or in a range of 0.1 T to 0.5 T.

18. 18. The method of claim 17, wherein the excitation of the first coil and the second coil is between 2000 and 8000 ampere-turns, and the excitation of the third coil is between −1 and −400 ampere-turns.

19. The method described in claim 13, wherein one of the first coil and the second coil is excited at a magnitude different from the magnitude of the excitation of the other coil.

20. The method described in claim 19, wherein the one coil is excited with a magnitude that is 0.5 to 2 times the magnitude of the excitation of the other coil.

21. 14. The method of claim 13, wherein a fourth coil is energized when forming the third magnetic lens at the sample plane, the fourth coil being energized in the same manner as the third coil.

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