Combined electrostatic-magnetic objective lens

Abstract

A combined electrostatic-magnetic objective lens located in a column of a device (1) having at least one charged particle beam, where the column further comprises a potential tube (16) positioned along an optical axis (18) of the column (2), wherein the lower end of the potential tube (16) comprises mechanically and electrically connected and at least partially electrically conductive segments, one or more of the segments being made of a paramagnetic or diamagnetic material and one or more segments (20) being made of a ferromagnetic material, wherein the first segment is made of a paramagnetic or diamagnetic material and each additional segment is made of a paramagnetic or diamagnetic material or a ferromagnetic material, wherein the first segment is located closer to the source (7) of electrons than the other segments.

Description

[0001] Combined Electrostatic-Magnetic Objective Lens

[0002] Technical Field

[0003] The invention relates to an implementation of an electrostatic-magnetic objective lens in devices using a charged particle beam, optimizing the resolution of the charged particle beam at a low accelerating energy.

[0004] Background of the Invention

[0005] Scanning electron microscopes (SEMs) are generally composed of a source of primary electrons producing an electron beam that then passes through at least one condenser lens and an aperture diaphragm, which together regulate the current of the primary particle beam, and subsequently passes through a system of scanning and centering elements and at least one objective lens, as described, e.g., in the patent CZ306807B6. This system of elements is referred to as the electron column.

[0006] Scanning electron microscopes may be used independently or within a system with other devices, e.g., with a device producing a focused ion beam (FIB). An advantage of these devices is the possibility of processing the sample using the FIB while simultaneously observing the sample using the SEM. Samples processed in this manner are suitable for observation, e.g., using a transmission electron microscope.

[0007] Objective lenses in SEMs are usually electromagnetic. An electromagnetic lens is composed of a coil through which current flows and a shell made of a ferromagnetic material, forming part of the magnetic circuit of the lens. At the point where the shell of the coil is interrupted (the so-called axial gap), a magnetic field arises between the so- called pole pieces. The shape, position, and dimensions of the pole pieces and the excitation of the coil determine the magnitude and shape of the magnetic field, which influences the electron beam. In the optimal design, the entire magnetic field except for the space of the axial gap is enclosed within the magnetic shell of the lens. The magnetic field in the material is represented by the magnetic induction vector and the magnetic flux.

[0008] In one possible embodiment, the electromagnetic lens may be so-called conventional one. A conventional lens has two pole pieces with an axial gap in which the magnetic field arises and locally influences the electron beam, wherein it does not substantially extend into the sample region. In another possible embodiment, the electromagnetic lens may be a so-called immersion one. An immersion lens may have either a single pole piece, where the magnetic field closes through the chamber of the microscope, or two pole pieces with a radial gap open toward the sample (in English, this type of lens is referred to as a snorkel lens). When using an immersion lens, the sample is immersed in the magnetic field generated by the lens. An advantage of the immersion lenses over the conventional ones is the reduction of optical defects (aberrations) and the improvement of the microscope’s resolution.

[0009] Electromagnetic lenses may also be replaced with electrostatic lenses that use electrodes instead of coils, or a combined lens may be created by combining coils, magnetic circuit, and electrodes.

[0010] For the resolving power of the SEM, the energy of the electrons passing through the electron column is also important. The electron energy is determined by the voltage supplied to the source of electrons and possibly by their acceleration in a potential tube, as will be described below. The electrons are produced by the source in such a way that an anode is positioned opposite the cathode of the source, and due to the high voltage between the cathode and the anode, the so-called accelerating voltage, electrons are emitted from the cathode and accelerated toward the anode. The accelerating voltage typically ranges in the tens of kV. However, the energy that the electrons gain in such an accelerating voltage can be too high for observing a number of samples, the resolution of such microscopes is lower, and the impact of high-energy particles may damage the sample.

[0011] That is why electron columns specialized for imaging the sample at a low accelerating voltage (below 3 kV) of electrons incident on the sample are increasingly used. However, this brings a number of technical problems. In the case of a low accelerating voltage, the influence of the magnetic field of the objective lens on the sample increases, resulting in a larger chromatic optical defect of the objective lens. Its reduction may be achieved, e.g., by combining an electromagnetic conventional objective lens with the application of a high negative voltage to the sample, as described, e.g., in the patent US7705302B2. The problem with this method is the incompatibility with topographic and tilted samples that distort the strong electrostatic field on the sample, which negatively influences the incident beam, and also the incompatibility with other devices working near the sample (e.g., nanomanipulators, gas injection systems, etc.).

[0012] Another possibility is the use of immersion objective lens as described, e.g., in the patent US6664544B1 . A disadvantage of this method is the impossibility of using magnetic samples and device accessories (e.g., a sample holder) and also the fact that the magnetic field of the immersion lens negatively influences the possible ion beam in a combined FIB-SEM device.

[0013] The third option is to use the so-called potential tube. It is multiple electrically conductive components arranged sequentially along the optical axis of the electron column such that the electron beam passes through them, to which a high positive voltage (about 10 kV) is applied, as described, e.g., in the patent US7425701 B2. Due to the high voltage in the potential tube, the electrons gain high energy and are decelerated at the very end of the potential tube, which is located in the objective lens. The deceleration of the electrons of the beam to the incident energy occurs in the electrostatic field generated by the difference in electric potential between the potential tube and the sample or additional electrode at a lower electric potential than that of the potential tube. The objective lens of such a device is also referred to as electrostatic-magnetic or combined because of the merging of the electrostatic and magnetic lens of the objective.

[0014] A disadvantage of the electrostatic-magnetic objective lens with closed poles without a magnetic field on the sample is the difficulty of optimizing the resolving power of the SEM at a low accelerating voltage, where in particular the influence of the chromatic defect of the objective lens needs to be reduced. The optimization of the objective lens consists in the merging of the magnetic and electrostatic lens such that when passing through the magnetic part of the objective lens, the electrons are still at high energy. The second measure is to bring the electrostatic and magnetic part of the objective lens as close to the sample as possible by extending the pole pieces of the objective lens towards the sample. However, this is limited by a lack of space in the vacuum chamber of the device and in the objective lens itself. The outer dimension of the objective lens must be kept in a certain shape and size such that it can be optimally combined with other common devices found in the vacuum chamber of the SEM, and in the case of a combined FIB- SEM device, additionally with the objective lens of the FIB.

[0015] A section through a conventional electrostatic-magnetic objective lens according to the prior art is shown in fig. 3. This objective lens comprises a coil, a shell, two pole pieces with an axial gap, the end of the potential tube, and an electrical insulation separating the potential tube from the other parts of the objective lens, which are grounded. The main plane of the magnetic part of the objective lens with closed poles is typically located at a distance greater than 10 mm from the end of the lower pole piece of the objective lens. For proper functioning of this objective lens, it is necessary to maintain sufficient thickness of both the pole pieces due to magnetic saturation and electrical insulation due to the high voltage of the potential tube.

[0016] Therefore, it would be desirable to come up with a new implementation of an electrostatic-magnetic conventional objective lens for a SEM that would improve the resolving power of the SEM when using a low accelerating voltage and optimally solve the above-mentioned problems related to the chromatic defect of such objective lens and the lack of space in the immediate vicinity of the objective lens.

[0017] Summary of the Invention

[0018] The above-mentioned problems are solved by a device having at least one charged particle beam comprising at least one column comprising a source of charged particles producing the charged particles, elements for shaping and directing the charged particles, an objective lens comprising a magnetic circuit comprising a coil, an upper magnetic pole, and a lower magnetic pole, where the column further comprises a potential tube positioned along the optical axis of the column, wherein the upper end of the potential tube is located behind the source of charged particles in the direction of the current of the charged particles and the lower end is located inside the objective lens between the upper magnetic pole and the lower magnetic pole of the objective lens.

[0019] The essence of the device lies in the fact that the lower end of the potential tube comprises mechanically and electrically connected and at least partially electrically conductive segments, one or more of the segments being made of a paramagnetic or diamagnetic material and one or more of the segments being made of a ferromagnetic material, the first segment is made of a paramagnetic or diamagnetic material and each additional segment is made of a paramagnetic or diamagnetic material or a ferromagnetic material, wherein the first segment is located closer to the source of electrons than the other segments. The segments are positioned such that more than 50 % of the magnetic flux of the objective lens magnetic circuit is closed through one or more segments made of ferromagnetic material.

[0020] An advantage of the present invention is shifting the magnetic part of the objective lens toward the sample, thereby optimizing the magnetic part of the objective lens and improving the resolving power of the SEM at a low accelerating voltage without having to interfere with the shell of the objective lens in both the space-limited vacuum chamber of the device and the limited space inside the objective lens.

[0021] Description of Drawings

[0022] A summary of the invention is further clarified using exemplary embodiments thereof, which are described with reference to the accompanying drawings. For the sake of clarity, only those parts of the device that are important in terms of the principle of the present invention are shown in the drawings.

[0023] Fig. 1 is a device with one column

[0024] Fig. 2 is a device with two columns

[0025] Fig. 3 is a section through the objective lens according to the prior art

[0026] Fig. 4 is a section through the objective lens of the present invention Fig. 5 is a section through the objective lens of the present invention with the marked shift of the magnetic field towards the sample

[0027] Exemplary Embodiments of the Invention

[0028] Said embodiments show exemplary variants of the embodiments of the invention, which, however, have no limiting effect from the point of view of the scope of protection.

[0029] The present method is implemented in a device 1_ with at least one charged particle beam. The device 1. is a scanning electron microscope (SEM), a microscope combining an electron beam and a focused ion beam (FIB-SEM), or another similar device using a charged particle beam.

[0030] In the first exemplary embodiment, the device 1. comprises a column 2 connected to a working chamber 4, inside which a device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. The column 2 contains a source 7 of electrons producing electrons, and further elements 8 for shaping and directing electrons into the beam, for example lenses, astigmatism correctors, diaphragms, and other commonly used components for particle optics (fig. 1 ).

[0031] In the second exemplary embodiment, the device 1. comprises two or more columns 2 with sources 7 of electrons connected to the working chamber 4.

[0032] In the third exemplary embodiment, the device 1. comprises at least one column 2 with the source 7 of electrons and at least one column 3 with a source 9 of ions connected to the working chamber 4 (fig. 2).

[0033] The column 2 with the source 7 of electrons is terminated by an objective lens 10. In the first exemplary embodiment of the objective lens 10, it is a combined objective lens that comprises a magnetic circuit that comprises a coil 11 through which current flows to generate a magnetic field that is conducted through a shell 12 of a ferromagnetic material (fig. 4). At the point where the shell 12 of the coil 11 is interrupted (a so-called axial gap 13), a magnetic field is generated between the upper pole piece 14 and the lower pole piece 15, which locally influences and focuses the beam of electrons. The size and position of the axial gap 13 determines the course of the focusing component of the magnetic field. In the second exemplary embodiment of the objective lens 10, it is an immersion objective lens 10 comprising the upper and lower pole piece 14, 15 having a magnetic field open toward the sample (the so-called snorkel lens).

[0034] A so-called potential tube 16 positioned along the optical axis 18 of the column is further located in the column 2 such that the beam of electrons passes through it. The potential tube 16 comprises multiple sequentially arranged, mechanically and electrically connected, and at least partially electrically conductive segments. In the first exemplary embodiment of the potential tube 16, it is positioned in the column 2 such that the upper end thereof is located behind the source 7 of electrons in the direction of the current of electrons and simultaneously forms an anode accelerating the electrons, and the lower end of the potential tube 16 is located inside the objective lens 10 in the axial gap 13 between the pole pieces 14, 15. In the second exemplary embodiment of the potential tube 16, the upper end thereof is located between the anode and the end of the upper pole piece 14 and the lower end of the potential tube 16 is located inside the objective lens 10 in the axial gap 13 between the pole pieces 14, 15.

[0035] A positive voltage of a magnitude of, e.g., 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. In the first exemplary embodiment of the column 2, the electrons of the beam are decelerated in an electrostatic field generated by the potential difference between the potential tube 16 and the sample, which is at a lower potential than the potential tube 16. The electrons of the beam are decelerated to an incident energy lower than 3 kV.

[0036] In the second exemplary embodiment of column 2, behind the lower end thereof in the direction of the current of electrons, one additional electrode is positioned or more additional electrodes are positioned at a lower electrical potential than that of the potential tube 16. The potential difference generates an electrostatic field that decelerates the electrons of the beam to an incident energy lower than 3 kV.

[0037] The magnetic and electrostatic fields focus the beam of electrons and thus determine the resolving power of the SEM. The best resolution may be achieved with the smallest possible width of the magnetic and electrostatic field in the direction of the optical axis 18 of the column 2 and, at the same time, when they are as close to the sample as possible. This can be achieved by reducing the axial gap 13, specifically by extending the upper pole piece 14 towards the sample. However, the objective lens 10 according to the state of the art does not allow this because an electrical insulation 17 and the end of the potential tube 16 are located in the given space of the objective lens 10. The course 21 of the magnetic field on the optical axis 18 of the magnetic part of the objective lens 10 according to the state of the art is shown in fig. 3.

[0038] The invention consists in a modification of the lower end of the potential tube 16. In the first exemplary embodiment of the potential tube 16, it is made of one at least partially electrically conductive segment of a paramagnetic or diamagnetic material at the point of the axial gap 13 such as, e.g., aluminum, titanium, copper, etc., or various alloys such as, e.g., bronze, and one segment 20 of a high-saturation ferromagnetic material. Both segments are mechanically and electrically connected to each other and arranged along the optical axis 8 of the column 2 such that the beam of electrons can pass through them.

[0039] In the second exemplary embodiment of the potential tube 16, two or more at least partially electrically conductive segments of a paramagnetic or diamagnetic material at the point of the axial gap 13, which are mechanically and electrically connected to each other and arranged along the optical axis 18 of the column 2 such that the beam of electrons can pass through them. Between these segments, the potential tube 16 is formed by one rotationally symmetrical segment 20 of a high-saturation ferromagnetic material, such that it is also mechanically and electrically connected to them and is also located on the axis 18 of the column 2 such that the beam of electrons may pass through it (fig. 4).

[0040] In the third exemplary embodiment of the potential tube 16, it is made of two or more at least partially electrically conductive segments of a paramagnetic or diamagnetic material at the point of the axial gap 13 and two or more segments 20 of a high-saturation ferromagnetic material such that all segments are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 such that the beam of electrons can pass through them, wherein their order is arbitrary.

[0041] In all the exemplary embodiments of the potential tube 16, the high-saturation ferromagnetic material is permendur, permalloy, and other materials having similar properties. All the segments of the potential tube 16 are at the same high potential of, e.g., 10 kV. In all the exemplary embodiments of the potential tube 16, a low magnetic resistance is maintained between the segment 20 or segments 20 of a ferromagnetic material and the upper pole piece 14, and more than 50 % of the magnetic flux passes through a part of the segment 20 or segments 20 of the ferromagnetic material located closer to the sample in the direction of the current of electrons. The part of the segment 20 or segments 20 of the potential tube 16 of the ferromagnetic material located closer to the sample in the direction of the current of electrons is then an indirect extension of the end of the upper pole piece 14 and, together with the lower pole piece 15, defines the axial gap 13. Thus, the invention allows to implement a magnetic lens with a narrower axial gap 13 without affecting the dimensions of the pole pieces 14, 15, the potential tube 16, and the electrical insulation 17. By narrowing the axial gap 13 in the direction toward the sample, the magnetic field of the objective lens 10 is shifted closer to the sample such that the principal plane represented by the maximum of the axial magnetic field of the objective lens 10 shifts closer to the sample compared to the objective lens 10 of the prior art. The course 21 of the magnetic field according to the prior art and the course 22 of the magnetic field according to the invention are shown in fig. 5. This will improve the resolving power of the SEM when using a low accelerating voltage.

[0042] In a specific exemplary embodiment, the device 1. comprises the column 2 connected to the working chamber 4, inside which the device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. In the column 2, a source 7 of charged particles producing electrons and also elements 8 for shaping and directing the electrons into the beam are located.

[0043] The column 2 is terminated by an electrostatic-magnetic objective lens 10. The objective lens 10 comprises a magnetic circuit that comprises the coil 11 through which current flows to generate a magnetic field that is enclosed by the shell 12 of the ferromagnetic material. In the axial gap 13, a magnetic field is generated between the upper and lower pole piece 14, 15 with the upper and lower magnetic poles.

[0044] The potential tube 16 positioned along the optical axis 18 of the column 2 is further located in the column 2 such that a beam of electrons passes through it. The upper end of the potential tube 16 is located behind the source 7 of electrons in the direction of the current of electrons and the lower end is located inside the objective lens 10 in the axial gap 13.

[0045] A positive voltage of a magnitude of 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. The electrons of the beam are then decelerated in the electrostatic field generated by the potential difference between the potential tube 16 and the grounded sample to an incident energy of 2 kV.

[0046] The potential tube 16 is made of two electrically conductive segments made of titanium at the point of the axial gap 13, which are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 so that a beam of electrons may pass through them. Between these segments, the potential tube 16 is formed by one segment 20 of the permendur material, such that it is also mechanically and electrically connected to them and is located on the axis 18 of the column 2 such that a beam of electrons may pass through it.

[0047] In this arrangement, a low magnetic resistance is maintained between the segment 20 made of the permendur material and the upper pole piece 14 and more than 50 % of the magnetic flux passes through the part of the segment 20 made of the permendur material located closer to the sample. The part of the segment 20 made of the permendur material located closer to the sample becomes an indirect extension of the end of the upper pole piece 14. This results in a narrowing of the axial gap 13 of the magnetic lens and a shift of the magnetic field of the objective lens 10 toward the sample, thereby improving the resolving power of the SEM.

[0048] In the second specific exemplary embodiment, the device 1. comprises two columns connected to the working chamber 4, inside which the device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. In the first connected column 2, a source 7 of charged particles producing electrons and also elements 8 for shaping and directing the electrons into the beam are located. In the second connected column 3, a source 9 of charged particles producing ions is located.

[0049] The column 2 is terminated by an electrostatic-magnetic objective lens 10. The objective lens 10 comprises a magnetic circuit that comprises the coil 11 through which current flows to generate a magnetic field that is enclosed by the shell 12 of the ferromagnetic material. In the axial gap 13, a magnetic field is generated between the upper and lower pole piece 14, 15 with the upper and lower magnetic poles.

[0050] The potential tube 16 positioned along the optical axis 18 of the column 2 is further located in the column 2 such that a beam of electrons passes through it. The upper end of the potential tube 16 is located behind the source 7 of electrons in the direction of the current of electrons and the lower end is located inside the objective lens 10 in the axial gap 13.

[0051] A positive voltage of a magnitude of 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. The electrons of the beam are then decelerated in the electrostatic field generated by the potential difference between the potential tube 16 and the grounded sample to an incident energy of 2 kV.

[0052] The potential tube 16 is implemented using five segments at the point of the axial gap 13, namely three electrically conductive segments made of a paramagnetic or diamagnetic material such as, e.g., aluminum, titanium, copper, bronze, etc. These segments are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 such that a beam of electrons may pass through them. Further, the potential tube 16 is composed of two segments of a ferromagnetic material such as, e.g., permalloy, permendur, etc., such that they are also mechanically and electrically connected to them and are located on the axis 18 of the column 2 such that a beam of electrons may pass therethrough. The segments are arranged such that the first segment is composed of a paramagnetic or diamagnetic material, the second segment is composed of a ferromagnetic material, the third segment is composed of a paramagnetic or diamagnetic material, the fourth segment is composed of a ferromagnetic material, and the fifth segment is composed of a paramagnetic or diamagnetic material, wherein the segments are arranged in sequence such that the first segment is located closest to the source 7 of electrons and the fifth segment is located furthest from the source 7 of electrons.

[0053] In this arrangement, a low magnetic resistance is maintained between the segments of a paramagnetic or diamagnetic material and the upper pole piece 14 and more than 50 % of the magnetic flux passes through the part of the segment made of the ferromagnetic material located closer to the sample. The part of the segment made of the ferromagnetic material located closer to the sample becomes an indirect extension of the end of the upper pole piece 14. This results in a narrowing of the axial gap 13 of the magnetic lens and a shift of the magnetic field of the objective lens 10 toward the sample, thereby improving the resolving power of the SEM.

[0054] In the third specific exemplary embodiment, the device 1 comprises the column 2 connected to the working chamber 4, inside which the device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. In the column 2, a source 7 of charged particles producing electrons and also elements 8 for shaping and directing the electrons into the beam are located.

[0055] The column 2 is terminated by an electrostatic-magnetic objective lens 10. The objective lens 10 comprises a magnetic circuit that comprises the coil 11 through which current flows to generate a magnetic field that is enclosed by the shell 12 of the ferromagnetic material. In the axial gap 13, a magnetic field is generated between the upper and lower pole piece 14, 15 with the upper and lower magnetic poles.

[0056] The potential tube 16 positioned along the optical axis 18 of the column 2 is further located in the column 2 such that a beam of electrons passes through it. The upper end of the potential tube 16 is located behind the source 7 of electrons in the direction of the current of electrons and the lower end is located inside the objective lens 10 in the axial gap 13.

[0057] A positive voltage of a magnitude of 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. The electrons of the beam are then decelerated in the electrostatic field generated by the potential difference between the potential tube 16 and the grounded sample to an incident energy of 2 kV.

[0058] The potential tube 16 is implemented using five segments at the point of the axial gap 13, namely two electrically conductive segments made of a paramagnetic or diamagnetic material such as, e.g., aluminum, titanium, copper, bronze, etc. These segments are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 such that a beam of electrons may pass through them. The potential tube 16 is further composed of three segments of a ferromagnetic material such as, e.g., permalloy, permendur, etc., such that they are also mechanically and electrically connected to them and are located on the axis 18 of the column 2 such that the beam of electrons may pass therethrough. The segments are arranged such that the first segment is composed of a paramagnetic or diamagnetic material, the second and third segments are composed of a ferromagnetic material, the fourth segment is composed of a paramagnetic or diamagnetic material, and the fifth segment is composed of a ferromagnetic material, wherein the segments are arranged in sequence such that the first segment is located closest to the source 7 of electrons and the fifth segment is located furthest from the source 7 of electrons.

[0059] In this arrangement, a low magnetic resistance is maintained between the segments of the ferromagnetic material and the upper pole piece 14 and more than 50 % of the magnetic flux passes through the part of the segment made of the ferromagnetic material located closer to the sample. The part of the segment made of the ferromagnetic material located closer to the sample becomes an indirect extension of the end of the upper pole piece 14. This results in a narrowing of the axial gap 13 of the magnetic lens and a shift of the magnetic field of the objective lens 10 toward the sample, thereby improving the resolving power of the SEM.

[0060] In the fourth specific exemplary embodiment, the device 1. comprises the column 2 connected to the working chamber 4, inside which the device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. In the column 2, a source 7 of charged particles producing electrons and also elements 8 for shaping and directing the electrons into the beam are located.

[0061] The column 2 is terminated by an electrostatic-magnetic objective lens 10. The objective lens 10 comprises a magnetic circuit that comprises the coil 11 through which current flows to generate a magnetic field that is enclosed by the shell 12 of the ferromagnetic material. In the axial gap 13, a magnetic field is generated between the upper and lower pole piece 14, 15 with the upper and lower magnetic poles.

[0062] The potential tube 16 positioned along the optical axis 18 of the column 2 is further located in the column 2 such that a beam of electrons passes through it. The upper end of the potential tube 16 is located behind the source 7 of electrons in the direction of the current of electrons and the lower end is located inside the objective lens 10 in the axial A positive voltage of a magnitude of 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. The electrons of the beam are then decelerated in the electrostatic field generated by the potential difference between the potential tube 16 and the grounded sample to an incident energy of 2 kV.

[0063] The potential tube 16 is made of three segments at the point of the axial gap 13, which are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 so that a beam of electrons may pass through them. The first and second segments are composed of a paramagnetic or diamagnetic material such as, e.g., aluminum, titanium, copper, bronze, etc., and the third segment is composed of a ferromagnetic material such as, e.g., permalloy, permendur, etc. The segments are arranged in sequence such that the first segment is located closest to the source 7 of electrons and the third segment is located furthest from the source 7 of electrons.

[0064] In this arrangement, a low magnetic resistance is maintained between segment 20 of the ferromagnetic material and the upper pole piece 14 and more than 50 % of the magnetic flux passes through the part of the segment made of the ferromagnetic material located closer to the sample. The segment made of the ferromagnetic material located closer to the sample becomes an indirect extension of the end of the upper pole piece 14. This results in a narrowing of the axial gap 13 of the magnetic lens and a shift of the magnetic field of the objective lens 10 toward the sample, thereby improving the resolving power of the SEM.

[0065] In the fifth specific exemplary embodiment, the device 1. comprises the column 2 connected to the working chamber 4, inside which the device 5 for positioning and holding the sample, at least one detector 6 of signal particles, and other commonly used components of the working chamber 4 are located. In the column 2, a source 7 of charged particles producing electrons and also elements 8 for shaping and directing the electrons into the beam are located.

[0066] The column 2 is terminated by an electrostatic-magnetic objective lens 10. The objective lens 10 comprises a magnetic circuit that comprises the coil 11 through which current flows to generate a magnetic field that is enclosed by the shell 12 of the ferromagnetic material. In the axial gap 13, a magnetic field is generated between the upper and lower pole piece 14, 15 with the upper and lower magnetic poles. The potential tube 16 positioned along the optical axis 18 of the column 2 is further located in the column 2 such that a beam of electrons passes through it. The upper end of the potential tube 16 is located behind the source 7 of electrons in the direction of the current of electrons and the lower end is located inside the objective lens 10 in the axial gap 13.

[0067] A positive voltage of a magnitude of 10 kV is applied to the potential tube 16, which accelerates the electrons exiting the source 7 towards the objective lens 10. The electrons of the beam are then decelerated in the electrostatic field generated by the potential difference between the potential tube 16 and the grounded sample to an incident energy of 2 kV.

[0068] The potential tube 16 is made of one electrically conductive segment made of titanium at the point of the axial gap 13 and one segment 20 of the permendur material, wherein the segment made of titanium is located closer to the source of electrons than the segment 20 of the permendur material. The segments are mechanically and electrically connected to each other and arranged along the axis 18 of the column 2 so that a beam of electrons may pass through them.

[0069] In this arrangement, a low magnetic resistance is maintained between the segment 20 made of the permendur material and the upper pole piece 14 and more than 50 % of the magnetic flux passes through the part of the segment 20 made of the permendur material located closer to the sample. The part of the segment 20 made of the permendur material located closer to the sample becomes an indirect extension of the end of the upper pole piece 14. This results in a narrowing of the axial gap 13 of the magnetic lens and a shift of the magnetic field of the objective lens 10 toward the sample, thereby improving the resolving power of the SEM.

[0070] List of Reference Signs

[0071] 1 - device

[0072] 2 - SEM column

[0073] 3 - FIB column

[0074] 4 - chamber

[0075] 5 - device for holding the sample

[0076] 6 - detector

[0077] 7 - source of electrons

[0078] 8 - elements for shaping and directing the beam

[0079] 9 - source of ions

[0080] 10 - objective lens

[0081] 11 - coil

[0082] 12 - shell

[0083] 13 - axial gap

[0084] 14 - upper pole piece

[0085] 15 - lower pole piece

[0086] 16 - potential tube

[0087] 17 - electrical insulation

[0088] 18 - optical axis of the column

[0089] 19 - optical axis of the second column

[0090] 20 - segment made of a ferromagnetic material

[0091] 21 - course of the magnetic field of the objective lens according to the prior art

[0092] 22 - course of the magnetic field of the objective lens according to the invention

Claims

CLAIMS1. A device (1 ) having at least one charged particle beam comprising at least one column (2) comprising a source (7) of charged particles producing the charged particles, elements (8) for shaping and directing the charged particles, an objective lens (10) comprising a magnetic circuit comprising a coil (11 ), an upper magnetic pole, and a lower magnetic pole, the column (2) further comprising a potential tube (16) positioned along an optical axis (18) of the column (2), wherein the upper end of the potential tube (16) is located behind the source (7) of charged particles in the direction of the current of the charged particles and the lower end is located inside the objective lens (10) between the upper magnetic pole and the lower magnetic pole of the objective lens (10), characterized in that the lower end of the potential tube (16) comprises mechanically and electrically connected and at least partially electrically conductive segments, one or more of the segments being made of a paramagnetic or diamagnetic material and one or more segments (20) being made of a ferromagnetic material, wherein the first segment is made of a paramagnetic or diamagnetic material and each additional segment is made of a paramagnetic or diamagnetic material or a ferromagnetic material, wherein the first segment is located closer to the source (7) of electrons than the other segments.

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