Combined column of a scanning electron microscope

Abstract

A combined column of a scanning electron microscope located in a device (1) with at least one beam of charged particles with at least one column (2) containing a source (7) of charged particles producing the charged particles, elements for shaping and directing the charged particles, an objective (13) at a potential forming an objective lens (15) formed by a magnetic field emanating onto the sample, and further a potential tube (18) at a potential different from the potential of the objective (13) positioned along the optical axis of the column (2), wherein the upper end of the potential tube (18) is located behind the source (7) of charged particles in the direction of the current of charged particles, and the lower end of the potential tube (18) is located in the column (2) behind the source (7) of charged particles in the direction of the current of charged particles and simultaneously in front of the objective lens (15) in the direction of the current of charged particles, wherein a deceleration electrostatic field generated as a result of the difference in electric potential between the potential tube (18) and the objective (13) is located between the lower end of the potential tube (18) and the objective lens (15).

Description

DescriptionTitle of Invention: Combined column of a scanning electron microscopeTechnical Field

[0001] The invention relates to an embodiment of the column of a scanning electron microscope, in particular to a device combining an electron and an ion beam. Background of the Invention

[0002] 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 beam of primary particles, and subsequently passes through a system of scanning and centering elements and through the objective, in which at least one objective lens is located, as described, e.g., in the patent CZ306807B6. This system of elements is referred to as the electron column.

[0003] Scanning electron microscopes may be used independently or in 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 monitoring the sample using the SEM. Samples processed in this manner are suitable for observation, e.g., by a transmission electron microscope.

[0004] The objectives of SEMs are usually electromagnetic. An electromagnetic objective is formed by a coil through which current flows and a ferromagnetic material shell that forms part of the magnetic circuit of the objective. A magnetic field (the objective lens) is generated between the so-called pole pieces, which affects the beam of electrons, at the point where the shell of the coil is interrupted (the so-called axial gap).

[0005] In one of the possible embodiments, the electromagnetic objective may be a so-called conventional one. A conventional objective has two pole pieces with an axial gap in which the magnetic field is generated and locally affects the electron beam, wherein it does not substantially extend into the sample region. In another one of the possible embodiments, the electromagnetic objective may be a so-called immersion one. An immersion objective may have either a single pole piece, where the magnetic field closes through the space of the chamber of the device into the upper part of the coil yoke, 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 the immersion objective, the sample is immersed in the magnetic field generated by the objective. An advantage of the immersion objectives over the conventional ones is the reduction of optical defects (aberrations) of the lens, and therefore a higher resolution of the microscope.

[0006] The electromagnetic objective may also be replaced with an electrostatic objective that uses electrodes instead of coils, or a combined objective may be created by combining coils, magnetic circuit, and electrodes.

[0007] The energy of the electrons passing through the electron column is also important for the resolving power of the SEM. 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 energies that the electrons gain in such an accelerating voltage can be too high for the observation of many samples, which negatively affects the image contrast, and the impact of high- energy particles may damage the sample.

[0008] 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.

[0009] In columns specialized for imaging the sample at a low accelerating voltage, the problem is the lower resistance of the electron beam to disturbances caused by external influences, such as, e.g., external electromagnetic fields, contaminants in parts of the column surrounding the beam, etc. Contaminants in the column cause charging of surfaces in the vicinity of the beam, undesired interactions of the electrons of the beam with the particles of the contaminants, and also increase the chromatic dispersion of the electrons of the beam due to space charge (the so-called Boersch effect). All these phenomena have a negative impact on the electron beam and cause deterioration of the electron beam spot.

[0010] In the case of a low accelerating voltage, the effect of the dispersion of the kinetic energy of the electrons in the electron beam also increases, resulting in a larger chromatic optical defect of the objective lens. Different solutions of this problem are used in the state of the art, with the aim of minimizing the chromatic defect.

[0011] One option is, e.g., to use a combination of electromagnetic conventional 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 in the vicinity of the sample (e.g. nanomanipulators, gas injection systems, etc.).

[0012] Another option is the use of the immersion objective (fig. 3), as described, e.g., in the patent US6664544B1. Electrons emitted from the source move through the column at a low kinetic energy (e.g., 1 keV), at which they remain until they hit the sample. The disadvantage of this method is the fact that the low-energy electron beam is more affected by the contaminants in the column and also by the space charge in the upper part of the column, which negatively affect the low-energy beam.

[0013] The third possibility is to use the so-called potential tube (fig. 4). This consists of 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 US7425701B2 or US5146090A. These parts as a whole are simply referred to as the potential tube. Electrons from the source, to which negative voltage is applied, are accelerated by positive voltage on the potential tube. As a result, they gain high energy, which they retain throughout the passage through the potential tube. The electrons of the beam are decelerated only at the very end of the potential tube, which is located in the objective. The deceleration of the electrons of the beam to the incident energy occurs in the electrostatic field (deceleration lens) 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, as described, e.g., in US4713543A. 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. A disadvantage of this method is the lower resolving power of the SEM compared to the resolution when an immersion objective is used and also the lower efficiency of the detection of signal particles.

[0014] The signal particles are mainly secondary electrons, i.e., electrons ejected from the sample due to the impact of the electron beam, and backscattered electrons, which are generated by the reflection of electrons from the electron beam when the beam hits the surface of the sample. The signal particles are detected by detectors positioned in the vacuum chamber of the SEM, in the objective, or in the column. The detector is, e.g., a scintillation detector.

[0015] The signal particles are detected in the device using the immersion objective mainly by means of a detector positioned in the objective, or further in the column. The signal particles move from the sample to the objective at a low energy. In the case of using a scintillation detector in the objective, the slow secondary electrons are deflected from the optical axis of the column and are accelerated toward the scintillation detector only after passing through the objective lens due to the high positive voltage (10 kV) applied to the metal layer on the surface of the scintillation detector. However, theeffect of this high positive voltage on the electrons of the electron beam on the optical axis of the column is negligible.

[0016] In a device using a potential tube, the signal particles are detected mainly by means of a detector positioned in the objective, or further in the column. The signal particles are directed upward into the objective by a small potential caused by a part of the field of the deceleration lens that penetrates to the sample. When passing through the objective toward the detector, they are then accelerated by the potential difference between the sample and the potential tube and hit the detector with this energy (e.g., 10 keV). When passing through the objective, a signal particle intersection typically occurs, the position of which determines the divergence of the signal particles and thus the yield of the detector.

[0017] A disadvantage of a column with a potential tube is the fact that the presence of the potential tube causes divergent upward motion of the signal particles through the objective, resulting in only a part of the signal electrons hitting the detector. Another disadvantage of the column with the potential tube is the reduction in the detection efficiency due to the topography of the observed sample, especially when the sample is tilted, because the topography of the sample affects the electrostatic extraction field below the objective.

[0018] In the prior art, solutions also exist that combine a column containing a potential tube ending in the objective, which is partially designed as an immersion one, where a part of the magnetic field from the objective penetrates the sample and a part is enclosed between the two poles of the objective. This combination is suitable for planar samples, but when observing topographic samples, the disadvantages of both the potential tube and the immersion objective are rather apparent. A disadvantage is also the difficulty of constructing such a column from a spatial point of view so that it can be combined in the chamber with a FIB column and other accessories necessary for FIB-SEM applications (e.g., nanomanipulators, gas injection systems, etc.).

[0019] Therefore, it would be useful to come up with a new design of the column for a SEM that would improve the resolving power of the SEM as well as the detection of secondary electrons when using a low accelerating voltage and when imaging all kinds of samples by combining the advantages of an immersion objective and a potential tube in the electron column.Summary of the Invention

[0020] The above shortcomings are solved by a device with at least one beam of charged particles containing at least one column containing a source of charged particles producing the charged particles, elements for shaping and directing the charged particles, an objective at a potential forming an objective lens formed by a magneticfield emanating onto the sample and further a potential tube at a potential different from the potential of the objective positioned along the optical axis of the column, having an upper and a lower end, wherein the upper end of the potential tube is located behind the source of charged particles in the direction of the current of charged particles, and the lower end of the potential tube is opposite the upper end of the potential tube, wherein the essence of the invention lies in the fact that the lower end of the potential tube is located in the column behind the source of charged particles in the direction of the current of charged particles and simultaneously in front of the objective lens in the direction of the current of charged particles, wherein a deceleration electrostatic field generated as a result of the difference in electric potential between the potential tube and the objective is located between the lower end of the potential tube and the objective lens.

[0021] In one of the variants, the lower end of the potential tube is located in the column behind the source of charged particles in the direction of the current of charged particles and simultaneously in front of the objective in the direction of the current of charged particles, wherein a deceleration electrostatic field generated as a result of the difference in electric potential between the potential tube and the objective is located between the lower end of the potential tube and the objective.

[0022] In a second one of the variants, the lower end of the potential tube is located in the column behind the source of charged particles in the direction of the current of charged particles and simultaneously in the objective in front of the objective lens in the direction of the current of charged particles, wherein a deceleration electrostatic field generated as a result of the difference in electric potential between the potential tube and the objective is located between the lower end of the potential tube and the objective.

[0023] The present invention solves the aforementioned shortcomings of the prior art by allowing to combine the advantages offered by the potential tube and the immersion objective. The high energy of the electron beam is maintained in the upper part of the column, which reduces the chromatic defect and increases the resistance of the electron beam to disturbing influences such as contaminants in the column, surface charging inside the column, external electromagnetic fields, etc. The deceleration of the electrons of the beam in front of the objective lens allows the use of an immersion objective, which creates an objective lens with the smallest chromatic and spherical defect and enables imaging of the sample at a high resolution, especially for FIB-SEM configurations. The use of an immersion objective allows for more efficient detection of signal particles, as no divergent propagation of the signal particles occurs. The low energy of the electrons of the beam with which they hit the sample also prevents rapid damage to the surface of the monitored sample.Description of Drawings

[0024] 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.

[0025] Fig. 1 is a device with one column

[0026] Fig. 2 is a device with two columns

[0027] fig. 3 is a column with an immersion objective lens according to the prior art

[0028] fig. 4 is a column with a potential tube according to the prior art

[0029] Fig. 5 is a column according to the present invention

[0030] Fig. 6 is another embodiment of a column according to the present inventionExemplary Embodiments of the Invention

[0031] 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.

[0032] The present method is implemented in the device 1 with at least one beam of charged particles. 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 beam of charged particles.

[0033] In the first exemplary embodiment, the device 1 comprises a column 2 with a source 7 of electrons 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 17, and other commonly used components of the working chamber (Fig. 1) are located.

[0034] 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.

[0035] 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 8 of ions connected to the working chamber 4 (Fig. 2).

[0036] In all the exemplary embodiments of the device 1, it comprises at least one detector 6 of signal particles 17. In the first exemplary embodiment of the detector 6 of signal particles 17, the detector 6 is located in the working chamber 4, in the second exemplary embodiment of the detector 6 of signal particles 17, the detector 6 is located in the column 2, and in the third exemplary embodiment of the detector 6 of signal particles 17, the detector 6 is located in the objective 13. The exemplary embodiments of the detectors 6 of signal particles 17 may be arbitrarily combined. The detector 6 may be, e.g., a scintillation detector.

[0037] The column 2 contains the source 7 of electrons emitting electrons. The source 7 of electrons contains a cathode 9, to which a high voltage HV, e.g., -200 V to -30 kV, is applied. Opposite the cathode 9 is an anode (a so-called extraction electrode 10). to which an extraction voltage, e.g., +4 kV floating on HV is applied, which extracts electrons from the cathode 9 and accelerates them toward the column 2.

[0038] The column 2 further contains a condenser lens 11 and a current diaphragm 12, which together define the current and the convergence angle of the electron beam incident on the sample.

[0039] The column 2 further contains deflectors for scanning with the electron beam.

[0040] The column 2 is terminated by a so-called immersion objective 13. The objective13 contains a magnetic circuit that contains a coil 14 through which current flows to generate a magnetic field, a so-called objective lens 15. which is guided by a shell made of ferromagnetic material. In the first exemplary embodiment of the objective 13. it contains one pole piece 16 and the magnetic circuit closes through the space of the chamber 4. In the second exemplary embodiment of the objective 13. it contains an upper and lower pole piece 16 having a magnetic field open toward the sample (so-called snorkel lens). The material of the objective 13 may be at an arbitrary potential, in an exemplary embodiment of the objective 13. the objective 13 is at ground potential.

[0041] The column 2 further contains a potential tube 18 positioned along the optical axis 19 of the column 2 such that a beam of electrons passes through it. A positive voltage having a magnitude of, e.g., 10 kV is applied to the potential tube 18. The upper end of the potential tube 18 is located behind the source 7 of electrons in the direction of the current of electrons. In an exemplary embodiment of the potential tube 18, its upper end is located behind the current diaphragm 12. The potential difference between the extraction electrode 10 of the source 7 of electrons and the potential tube 18 causes further acceleration of the electrons exiting the source 7, with kinetic energy given by the extraction voltage, toward the objective 13. The electrons of the beam retain the high energy throughout the entire passage through the potential tube 18 and are decelerated at its lower end.

[0042] The lower end of the potential tube 18 according to the invention is positioned in the column 2 behind the source 7 of electrons in the direction of the current of electrons. The distance of the lower end of the potential tube 18 from the source 7 of electrons is greater than the distance of the upper end of the potential tube 18 from the source 7 of electrons. In the first exemplary embodiment of the potential tube 18, its lower end is located in front of the immersion objective 13 in the direction of the current of electrons (Fig. 5), in the second exemplary embodiment of the potential tube 18. its lower end is located in the immersion objective 13 such that the deceleration field isgenerated in front of the objective lens 15 in the direction of the current of electrons (Fig. 6).

[0043] The deceleration field (so-called deceleration lens 21) is generated by the potential difference between the potential tube 18 and the objective 13. The deceleration lens 21 reduces the kinetic energy of the electrons of the beam (e.g., 11 keV) to the incident energy (e.g., 1 keV). The electrons with this low energy subsequently pass through the objective lens 15 and hit the sample.

[0044] In a specific exemplary embodiment of the device 1, this is a combined FIB-SEM device that contains one SEM column 2 with the source 7 of electrons and one FIB column 3 with the source 8 of ions. Both columns 2, 3 are connected to the working chamber 4, inside which the device 5 for positioning and holding the sample is located. A scintillation detector 6 is located in the SEM column 2 to detect secondary and backscattered electrons (signal particles 17).

[0045] The source 7 of electrons contains the cathode 9, to which the high voltage HV is applied -1 kV. Opposite the cathode 9 is the extraction electrode 10, to which an extraction voltage of +4 kV floating on the voltage of the cathode 9 is applied, which extracts electrons from the cathode 9 and accelerates them toward the column 2.

[0046] The column 2 further contains the condenser lens 11, the current diaphragm 12, and deflectors for scanning with the electron beam.

[0047] The column 2 further contains the immersion objective 13 with one pole piece 16 generating a magnetic field. The sample is positioned in the chamber 4 of the device so as to be located in this magnetic field.

[0048] The column 2 further contains a potential tube 18 positioned along the optical axis 19 of the column 2 such that a beam of electrons passes through it. A positive voltage having a magnitude of 10 kV is applied to the potential tube 18. The upper end of the potential tube 18 is located behind the source 7 of electrons and behind the current diaphragm 12 in the direction of the current of electrons. The potential difference between the extraction electrode 10 of the source 7 of electrons and the potential tube 18 causes further acceleration of the electrons exiting the source 7 toward the objective lens 15. In this part of the column 2, the kinetic energy of the electrons is 11 keV.

[0049] The lower end of the potential tube 18 is located behind the source 7 of electrons in the direction of the current of electrons and simultaneously in front of the objective 13. which is at ground potential. The potential difference between the potential tube 18 and the objective 13 generates an electrostatic field (deceleration lens 21), in which the kinetic energy of the electrons of the beam is reduced to the incident energy of 1 keV. The electrons of the beam with this low energy subsequently pass through the objective lens 15 and hit the sample.List of Reference Signs

[0050] 1. device2. column with the source of electrons3. column with the source of ions4. chamber5. device for holding the sample6. detector7. source of electrons8. source of ions9. cathode10. extraction electrode11. condenser lens12. current diaphragm13. objective14. coil15. objective lens16. pole piece17. signal particles18. potential tube19. optical axis of the SEM column20. optical axis of the FIB column21. deceleration lens

Claims

Claims

1. A device (1) with at least one beam of charged particles comprising at least one column (2) containing a source (7) of charged particles producing the charged particles, elements for shaping and directing the charged particles, an objective (13) at a potential forming an objective lens (15) formed by a magnetic field emanating onto the sample, and further a potential tube (18) at a potential different from the potential of the objective (13) positioned along the optical axis of the column (2) and having an upper and a lower end, wherein the upper end of the potential tube (18) is located behind the source (7) of charged particles in the direction of the current of charged particles and the lower end of the potential tube (18) is opposite the upper end of the potential tube (18), characterized in that the lower end of the potential tube (18) is located in the column (2) behind the source (7) of charged particles in the direction of the current of charged particles and simultaneously in front of the objective lens (15) in the direction of the current of charged particles, wherein a deceleration electrostatic field (21) generated as a result of the difference in electric potential between the potential tube (18) and the objective (13) is located between the lower end of the potential tube (18) and the objective lens (15).

2. The device (1) with at least one beam of charged particles according to claim 1, characterized in that the lower end of the potential tube (18) is located in the column (2) behind the source (7) of charged particles in the direction of the current of charged particles and simultaneously in front of the objective (13) in the direction of the current of charged particles, wherein a deceleration electrostatic field (21) generated as a result of the difference in electric potential between the potential tube (18) and the objective (13) is located between the lower end of the potential tube (18) and the objective (13).

3. The device (1) with at least one beam of charged particles according to claim 1, characterized in that the lower end of the potential tube (18) is located in the column (2) behind the source (7) of charged particles in the direction of the current ofcharged particles and simultaneously in the objective (13) in front of the objective lens (15) in the direction of the current of charged particles, wherein a deceleration electrostatic field (21) generated as a result of the difference in electric potential between the potential tube (18) and the objective (13) is located between the lower end of the potential tube (18) and the objective (13).

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