JP6074760B2 - Electron beam irradiation device - Google Patents

Description

  The present invention relates to an electron beam irradiation apparatus.

  In an electron microscope, Koehler illumination is known as a method of irradiating a sample with an electron beam in parallel.

  Conventionally, Koehler illumination is considered as the most effective method for increasing the parallelism of a beam. The principle of this illumination method is described in Non-Patent Document 1 issued in 1893 (Non-Patent Document 2 is an English translation thereof). Of course, at that time, an electron microscope was not yet invented, and this illumination method was proposed for an optical microscope.

  Later, the electron microscope sample irradiation method was introduced at the International Electron Microscope Conference held in 1990 (Non-Patent Document 3), and this method was further developed and applied to the electron microscope in conjunction with Koehler illumination. It was presented at the American Electron Microscopy Society (EMSA) in 1991 (Non-Patent Document 4).

  FIG. 1 is a diagram illustrating a simulation result of an electron trajectory by Koehler illumination. Here, the simulation result of the electron trajectory from the front focal plane of an objective lens (not shown) to the sample is shown. A drawing having the same contents as FIG. 1 is also published in Non-Patent Document 1 and Non-Patent Document 3.

  In FIG. 1, the arrow on the left is a light source image (that is, a crossover image of electron beams) 1 created from a not-shown electron gun (light source) by a condenser lens (not shown). In Koehler illumination, the irradiation lens system is such that this image 1 is located at the front focal plane (focal length f) of the objective lens and the sample plane 3 is located at the rear focal plane (focal length f) of the objective lens. Composed.

  As shown in FIG. 1, a beam (hereinafter also referred to as an “axial beam”) emitted from the center of the image 1 (that is, the center of the light source) is parallel by the objective lens even though it is emitted at various angles. It becomes a beam and irradiates the sample surface 3 uniformly. From FIG. 1, it can be seen that the beam divergence angle from the image 1 (that is, the light source) is an area of beam divergence on the sample surface 3. That is, the beam emitted at a large angle irradiates the sample surface 3 widely.

On the other hand, a beam (hereinafter also referred to as an “off-axis beam”) that deviates from the center of image 1 (that is, the center of the light source) has the same angle as the on-axis beam that protrudes from the center of image 1. Although the beam (that is, the beam parallel to the axial beam) reaches the same position as the axial beam on the sample surface 3, it can be seen that the position is irradiated obliquely (that is, with an angle). That is, as long as the image 1 (that is, the light source) has a finite size (2d ₁ diameter), a perfect parallel beam cannot be produced, and the beam having an angle determined by the size of the image 1 (light source) 3 (irradiation range size = 2d ₂ diameter). Therefore, a perfect collimated beam cannot be produced by Koehler illumination. In FIG. 1, “2α ₁ ” is a divergence angle of a beam emitted from an arbitrary point on the image 1 (half angle α ₁ is an opening angle), and “2α ₂ ” is a constant divergence angle (2α ₁ ) is an angle range in which a beam emitted from an arbitrary point on the image 1 irradiates the sample surface 3 (half angle α ₂ is an opening angle).

However, in most of the literatures on Koehler illumination, the Koehler illumination has been introduced as an illumination method for obtaining a parallel beam, although a diagram having the same content as FIG. 1 is drawn. Conventionally, the demand for beam parallelism is not so strong, and when a field-emission electron gun (FEG) or the like is used, the light source size (light source diameter) is small ( In other words, it is close to a so-called point light source), and since the divergence angle of the beam that irradiates the sample is small, it is considered that the beam is almost parallel. That is, if the required degree of parallelism is 10 ⁻³ rad (1 mrad) or at most 10 ⁻⁴ rad (0.1 mrad), the beam shown in FIG. 1 can be called a parallel beam. .

However, for example, in an irradiation lens system that requires high parallelism of the order of 10 ⁻⁵ , such as Diffractive Imaging, Koehler illumination can no longer be considered as a method for obtaining a parallel beam. .

  Diffraction imaging is one of the methods for realizing high resolution in an electron microscope, and has recently attracted a great deal of attention, and advanced research is being developed worldwide. Diffraction imaging does not require a lens for imaging, and it can realize imaging with diffraction-limited resolution even for non-crystalline materials that do not have periodicity despite being based on a diffraction pattern. Electron microscopy. In diffraction imaging in an electron microscope, it is an important issue to irradiate a sample in parallel and in a minute region with an electron beam.

A. Kohler, "Ein neies Beleuchtungsverfahren fur mikrophotographische Zwecke", Zeitschrift fur wissenschaftl. Mikroskopie, vol. 10, pp. 433-440, 1893 "A new system of illumination for photomicrographic purposes," translated by P. Evennett, Proceedings RMS vol. 28/4, 1993 G. Benner, J. Bihr, and M. Prinz, "A new illumination system for an analytical transmission electron microscope using a condenser objective lens," Proceedings of the XIIth International Congress for Electron Microscopy, San Francisco Press, pp. 138-139 , 1990 W. Probst, R. Bauer, G. Benner, and J.L. Lehman, "Koehler illumination advantage for imaging in TEM," EMSA, San Francisco Press, pp. 1010-1011, 1991 K. K. Christenson and J. A. Eades, "parallel illumination in the transmission electron microscope," Ultramicroscopy, 19, pp. 191-194, 1986 D. Eyidi, C. Hebert and P. Shattschneider, "Short note on parallel illumination in the TEM," Ultramicroscopy, 106, pp. 1144-1149, 2006

  According to Non-Patent Document 1 (and Non-Patent Document 2 which is an English translation thereof), in the age of Koehler, light bulbs were not yet widespread as light sources for irradiating samples in optical microscopes, and they were brightened by flames. The sample was not uniformly irradiated, and there was uneven illumination. Koehler lighting was the only lighting method devised to eliminate this uneven illumination. Koehler illumination is proposed as a method in which a beam emitted from one point on the light source illuminates the entire sample, and one point on the sample is illuminated from all points on the light source. Koehler has not touched on the parallel beam at all, and has proposed Koehler lighting as a method for realizing uniform illumination.

  Here, the description that “a beam emitted from one point on the light source illuminates the entire sample” can be read as an axial beam (that is, a beam emitted from the center of the light source) becomes a parallel beam. However, the explanation that “one point on the sample is illuminated from all points on the light source” means that an off-axis beam (ie, a beam emitted from a point other than the center of the light source) It can be read as illuminating at various tilt angles.

  Therefore, for example, in diffraction imaging in which a parallel beam is required to irradiate the entire observation region of the sample as described above, another illumination method that is not Koehler illumination is required.

  On the other hand, in the field of electron microscopes, fields that have so far required illumination with high parallelism are electron diffraction and Lorentz electron microscopes.

  Regarding electron diffraction, although there are a few literatures that consider the generation of parallel beams that irradiate the sample (Non-patent Documents 5 and 6), these papers do not mention Koehler illumination, and are based on magnetic fields. It only mentions that parallelism is disturbed by rotation.

  The Lorentz electron microscope requires a parallel beam, but there are no papers that discuss it. The Lorentz electron microscope creates a domain wall image by defocusing, and the sharpness of the domain wall image improves as the parallelism of the beam increases.

  Therefore, a method used to increase the parallelism of the beam in the Lorentz electron microscope is a final stage condenser lens (for example, a second stage second condenser lens in the case of a two-stage configuration. In this case, the excitation is weakened. This corresponds to enlarging the image of the light source created by the second condenser lens. And this is the same as Koehler illumination expanding the beam by the last condenser lens. However, the Lorentz electron microscope observes the magnetic domain structure of a sample. Therefore, in the Lorentz electron microscope, in order to avoid applying a magnetic field to the sample, the objective lens is used off, or even if the objective lens is used, only the imaging lens system is provided, and the sample and the front thereof are provided. An objective lens without a magnetic field, that is, without an irradiation lens is used. Therefore, the sample is located far from the second condenser lens, and a beam that is reduced by the first condenser lens in the first stage is enlarged and projected by the second condenser lens, thereby obtaining a beam that is as parallel as possible. I am doing so.

  This method does not particularly use the Koehler illumination, but uses the second condenser lens weak excitation condition including the Koehler illumination condition. This method has been widely used before Koehler illumination for an electron microscope is introduced in Non-Patent Document 3 and Non-Patent Document 4. Of course, Kohler's proposal for an optical microscope is long before the Lorentz electron microscope started. In other words, it can be said that the Lorentz electron microscope is a method that stipulates only the excitation condition of the second condenser lens that controls the beam irradiated on the sample. No conditions are defined for the first condenser lens in the previous stage of the second condenser lens.

  When considering Kohler illumination with a three-stage irradiation lens system (CL1 + CL2 + OL) in which a condenser lens (CL1, CL2) having a two-stage configuration and an objective lens (OL) are combined, the Koehler illumination determines the excitation conditions of the second condenser lens. The second condenser lens is realized by fitting so that the image plane of the second condenser lens coincides with the front focal plane of the objective lens. For example, in a spot mode used in SEM (Scanning Electron Microscope) or the like, the image of the second condenser lens is immediately behind the second condenser lens, and the beam diameter is reduced. Then, the beam diverged from the beam enters the irradiation system of the objective lens, and the further reduced beam strikes the sample. On the other hand, in Koehler illumination, the excitation of the second condenser lens is weaker than in the spot mode, and the image of the second condenser lens is brought closer to the objective lens in order to coincide with the front focal plane of the objective lens. For this reason, in Koehler illumination, the second condenser lens is often used in an enlargement mode. This is because the focal length of the objective lens is usually short because a high-performance objective lens is used. Therefore, the front focal plane of the objective lens is located immediately in front of the objective lens far from the second condenser lens. Because it is. In addition, a number of coils such as a deflection system, astigmatism correction, and a beam scan system are arranged between the second condenser lens and the objective lens, and a large space is required to accommodate them. This is one reason. Thus, it has been considered that the beam is collimated when the image plane position of the second condenser lens coincides with the front focal plane of the objective lens. The above is generally the case where the on-axis beam is collimated when the configuration of the irradiation lens system such as TEM (Transmission Electron Microscope) or SEM satisfies the conditions of Koehler illumination. This is the reason why the beam is illuminated with an angle.

This can be described quantitatively as follows. For example, in FIG. 1, beams emitted at an angle of 5 mrad from the center of image 1 (X = 0) and both ends of image 1 (X = ± 0.5 μm) enter the objective lens and satisfy the conditions of Koehler illumination. When the light beam comes out under such excitation conditions, the tilt angle with respect to the optical axis 5 of the on-axis beam (X = 0) becomes a value of 3.66002 × 10 ⁻⁶ rad and has a parallelism of the order of 10 ^−6. A beam is realized. On the other hand, for the off-axis beam (X = ± 0.5 μm), the inclination angles with respect to the optical axis 5 are values of −4.19731 × 10 ⁻⁴ rad and 4.24727 × 10 ⁻⁴ rad, respectively. Although the parallelism is an order of magnitude higher than the angle (5 × 10 ⁻³ rad) when emitted from both ends of the image 1, the parallelism is two orders of magnitude lower than the axial beam. Of course, this value is attributed to the fact that the light source has a finite size as described above. Therefore, if the light source size is an order of magnitude smaller by ± 50 nm, parallelism of the order of 10 ⁻⁵ is realized. It will be.

For example, in an electron gun such as a field emission electron gun (FEG), the light source size is about 10 nm. Therefore, originally, parallelism on the order of 10 ⁻⁵ should be sufficiently realized only by Koehler illumination. However, in actual experiments, there is no evidence that parallelism on the order of 10 ⁻⁵ has been realized. This is considered to be because the actual light source size is increased due to aberrations such as the acceleration lens when accelerating electrons and the irradiation lens system thereafter, the Bercher effect, and the like. Therefore, by introducing an aberration correction device, there is a possibility that these aberrations can be reduced and a light source size of about 10 nm can be maintained. However, the introduction of the aberration correction device further requires cost and space. In the first place, when an electron gun having a large light source size is used, the introduction of the aberration correction device itself does not make much sense.

  Therefore, there is a demand for a technique that can increase the parallelism of the beam only by devising the existing irradiation lens system even when the light source size is large.

  In view of the above points, the present inventor has improved the light source size, that is, the case where the beam diameter of the light source is large, by improving the usage of the condenser lens system among the irradiation lens systems. We have devised irradiation conditions that can bring inferior parallelism to off-axis beams.

  An object of the present invention is to add an electron beam that can increase the parallelism of a beam without adding any new device other than the shape and relative positional relationship of the lens, and even when the light source size is large. It is to provide an irradiation apparatus.

  The electron beam irradiation apparatus of the present invention has an electron beam source that generates an electron beam, and an irradiation lens system that collects the generated electron beams and applies them to a sample, and the irradiation lens system includes a plurality of lenses, Among the plurality of lenses, the lens immediately preceding the last-stage lens is set so that the magnification is set to 1 or less and its image plane coincides with the front focal plane of the last-stage lens. Yes.

  When the irradiation lens system includes one or more condenser lenses that collect the generated electron beams and an objective lens that applies the electron beams collected by the condenser lenses to the sample, the one or more condenser lenses The condenser lens at the final stage is arranged so that the magnification is set to 1 or less and the image plane thereof coincides with the front focal plane of the objective lens.

  When the irradiation lens system has a plurality of condenser lenses and irradiates a sample with an electron beam using the plurality of condenser lenses without using an objective lens, the last condenser lens of the plurality of condenser lenses The immediately preceding condenser lens is arranged such that the magnification is set to 1 or less and the image plane thereof coincides with the front focal plane of the final condenser lens.

Preferably, the excitation condition of the one-stage lens is adjusted so that the distance between the crossover position of the electron beam source and the image plane of the one-stage lens is short. Specifically, for example, the distance between the crossover position of the electron beam source and the image plane of the previous lens is d ₁ , and the crossover position of the electron beam source and the main plane of the previous lens Is d ₂ , and the distance between the crossover position of the electron beam source and the main plane of the last lens is d ₃ , the following relationship is further satisfied.
d ₁ ≦ d ₂ + (d ₃ −d ₂ ) / 2

  According to the present invention, by improving the usage of the irradiation lens system, it is possible to add a beam without using any new device except for the shape of the lens and the relative positional relationship, and even when the light source size is large. The parallelism can be increased.

The figure which shows the simulation result of the electron orbit by Koehler illumination Schematic which shows an example of the irradiation lens system which comprises the electron beam irradiation apparatus which concerns on one embodiment of this invention Schematic showing the structure of the electron lens constituting the irradiation lens system of FIG. The figure which shows the simulation result of the electron orbit when the magnification of the last condenser lens is set to a certain value The figure which shows the simulation result of the electron orbit when the magnification of the condenser lens of the last stage is set to other values The figure which shows the simulation result of the electron orbit when the magnification of the condenser lens of the last stage is set to another value The figure which shows the simulation result of the electron orbit when the magnification of the condenser lens of the last stage is set to another value Diagram showing the trajectory of the on-axis beam and off-axis beam when the magnification of the final condenser lens is changed A graph showing the relationship between the focal position of the objective lens and the beam irradiation angle of the off-axis beam onto the sample Graph showing the relationship between the focal position of the objective lens and the magnification of the condenser lens Graph showing the relationship between the focal position of the objective lens and the beam radius on the sample Diagram for explaining the case where the distance between the objective lens and the last condenser lens is changed The figure which shows the structure which shortened the distance of the pole piece of the last condenser lens and the pole piece of the objective lens The figure for demonstrating the case where a double gap lens is used in order to eliminate image rotation Schematic diagram showing an example of an optical system of a diffraction imaging apparatus as a first application example of an electron beam irradiation apparatus according to the present embodiment Ray diagram of the diffraction imaging apparatus of FIG. Schematic which shows an example of the optical system of a Lorentz electron microscope as a 2nd application example of the electron beam irradiation apparatus which concerns on this Embodiment Ray diagram of Lorentz electron microscope in FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The present invention relates to an electron beam irradiation apparatus widely used for an irradiation lens system of an electron microscope, and more particularly to, for example, an electron beam irradiation apparatus suitable for electron diffraction imaging, and more specifically to improve image quality. The present invention relates to the position and shape of an electron lens for obtaining a parallel beam, drive current, and the like. In addition, the irradiation lens system may or may not use an objective lens. As will be described later, the present invention irradiates a sample with a condenser lens and an objective lens. In addition, the present invention can also be applied to a case where a sample is irradiated with a beam using only a condenser lens without using an objective lens.

  FIG. 2 is a schematic view showing an example of an irradiation lens system constituting the electron beam irradiation apparatus according to the embodiment of the present invention. The example of FIG. 2 is a case where a sample is irradiated with a beam using a condenser lens and an objective lens. Here, for simplicity of explanation, a three-stage irradiation lens system (CL1 + CL2 + OL) in which a two-stage condenser lens (CL1 + CL2) and an objective lens (OL) are combined will be described as an example.

  An electron beam irradiation apparatus 10 shown in FIG. 2 is collected by an electron gun (electron beam source) 20 that generates an electron beam, a condenser lens 30 that collects the generated electron beam, and a condenser lens 30 as an irradiation lens system 12. And an objective lens (OL) 40 that applies an electron beam to the sample. The condenser lens 30 has a two-stage configuration, and includes a first condenser lens (CL1) 32 at the first stage and a second condenser lens (CL2) 34 at the second stage. In the electron gun 20, a crossover 22, which is a portion of the minimum cross section where an electron beam emitted from a cathode (not shown) is narrowed, is formed. The electron beam emitted from the electron gun 20 sequentially passes through the first condenser lens (CL1) 32, the second condenser lens (CL2) 34, and the objective lens (OL) 40, and irradiates the sample 50.

In this irradiation lens system 12, the condenser lens (CL 2) 34 at the final stage (that is, the second stage here) of the condenser lens (CL 1 + CL 2) 30 has a magnification in order to increase the parallelism of the beam irradiated to the sample 50. (= B / a) is set to 1 or less, and the image plane thereof is arranged to coincide with the front focal plane (focal length f) of the objective lens (OL) 40. Preferably, in addition to this condition, the final stage (2) is set so that the distance between the crossover position in the electron gun 20 and the image plane of the final stage (second stage) condenser lens (CL2) 34 is shortened. The excitation condition of the condenser lens (CL2) 34 in the (stage) is adjusted. Specifically, the electron beam irradiation apparatus 10 determines the distance between the position P _{0 of the} crossover 22 in the electron gun 20 and the position P ₃ of the image plane of the final stage (second stage) condenser lens (CL 2) 34. d _im is the distance between the position P _{0 of the} crossover 22 in the electron gun 20 and the position P ₂ of the main plane of the condenser lens (CL2) 34 in the final stage (second stage) d _CL2 , and the cross in the electron gun 20 when the distance between the position _{P 0} and the objective lens (OL) position _{P 4} of the main plane 40 of over 22 was _{d OL,} further, the following relationship,
d _im ≦ d _CLn + (d _OL −d _CLn ) / 2
It is configured to satisfy. In FIG. 2, “P ₁ ” is the position of the image plane of the first stage condenser lens (CL 1) 32, and “P ₅ ” is the last stage condenser lens (CL 2) 34 and objective lens (OL) 40. Is the middle point. The principle of the present invention will be described later in detail.

  The condenser lenses (CL1, CL2) 32 and 34 and the objective lens (OL) 40 are, for example, electron lenses (magnetic field type electron lenses) that focus an electron beam by a magnetic field, for example.

  FIG. 3 is a schematic view showing the structure of an electron lens constituting the irradiation lens system of FIG. Here, for the sake of simplification, a second condenser lens (CL2) 34 and an objective lens (OL) 40 in the final stage (second stage) constituting the irradiation lens system 12 of FIG. 2 are shown. Although not shown, the structure of the first condenser lens (CL1) 32 is the same as that of the second condenser lens (CL2) 34 and the objective lens (OL) 40. Moreover, in the following description, the same code | symbol is attached | subjected to the same member among the structural members of an electronic lens regardless of a condenser lens or an objective lens.

  As shown in FIG. 3, the electron lens includes a coil 60 that generates a magnetic field and a metal frame (yoke) 62 made of a material having high magnetic permeability (for example, iron). The electron lens has a rotationally symmetric shape about the optical axis 70 through which the electron beam passes. The pole piece 64 is also called a pole piece, and a high quality material may be used as compared with the yoke 62, and is processed with high accuracy. Here, the position of the pole piece is defined as the center position between the upper pole and the lower pole of the pole piece.

  In the electron lens, the electrons move in a spiral shape, so the image rotates. In the electron lens, the focal length can be changed by changing the current (excitation current) flowing through the coil 60.

  In the configuration of FIG. 2, in particular, the objective lens (OL) 40 is preferably a magnetic field-free electron lens that does not apply a magnetic field to the sample 50. For example, the objective lens (OL) 40 has a shape similar to that of an objective lens for a scanning electron microscope (SEM), and an ordinary objective lens for SEM can be used.

  Here, the principle of the present invention will be described. Here, a two-stage irradiation lens system is considered as a more general configuration. In this two-stage irradiation lens system, for example, the front lens is the final condenser lens CLn, and the rear lens is the objective lens OL. When applied to the irradiation lens system 12 of FIG. 2, the front lens corresponds to the second condenser lens (CL2) 34, and the rear lens corresponds to the objective lens (OL) 40 (see FIG. 3).

  When the sample is irradiated with the beam using only the condenser lens without using the objective lens, the two-stage condenser lens serves as the last stage condenser lens CLn and the objective lens OL. For example, in this case, when applied to an irradiation lens system 204 of FIG. 17 described later, the front lens corresponds to the first condenser lens (CL1) 32, and the rear lens corresponds to the second condenser lens (CL2) 34. To do.

  In such a two-stage irradiation lens system, a beam having a high degree of parallelism can be obtained by setting the magnification of the front lens (final stage condenser lens CLn) to 1 or less as described above, thereby crossing the front lens. Over (diffractive pattern formed at the focal point) is brought close to the image in distance, and the excitation condition of the rear lens (objective lens OL) is the same as the front focal plane of the rear lens. Obtained when set to do.

  This condition includes the Koehler illumination condition. The condition of the Koehler illumination is a condition necessary for collimating the on-axis beam, and there is no condition for collimating the beam under a condition that does not satisfy this condition. The condition in question here is the condition for collimating the off-axis beam. The condition for collimating the off-axis beam includes the condition for collimating the on-axis beam, and further requires additional conditions.

  In the electron microscope, normally, the lens immediately before the objective lens OL (condenser lens CLn at the final stage) is used in the magnifying system for the convenience of the apparatus configuration. For this reason, off-axis beams are not collimated in Koehler illumination. If the magnification of the last stage condenser lens CLn is set to the reduction mode, the beam crossover of the last stage condenser lens CLn is located very close to the image plane, and not only the image plane of the on-axis orbit but also the cross of the off-axis orbit. The over surface can also be placed very close to the front focal plane of the objective lens OL. This is the principle of the present invention.

  For example, in an irradiation lens system such as a TEM (transmission electron microscope) or SEM (scanning electron microscope), the distance between the last condenser lens CLn positioned in front of the objective lens OL and the objective lens OL is large. Yes. This is because a deflection system, an astigmatism correction system, a scan system, and a sample insertion system were further inserted in the old TEM between the last condenser lens CLn and the objective lens OL, which required a large space. is there. Further, as the objective lens OL, a lens having high performance, that is, a short focal length is generally used. Therefore, the front focal plane of the objective lens OL is usually close to the objective lens OL. is there. Therefore, in an electron microscope, the distance between the last-stage condenser lens CLn and the front focal plane of the objective lens OL is always long. Therefore, the last-stage condenser lens CLn is generally used as an enlargement system. It is. In other words, the condenser lens CLn at the final stage is not an enlargement system because of the positive reason that the enlargement system is desirable in terms of electro-optics. It is thought that.

  In an SEM or the like, an irradiation lens system is used as a reduction system in order to scan a sample with a narrowly focused beam. Therefore, it is likely that the final stage condenser lens CLn is naturally used in the reduction system. However, as can be seen by examining the positional relationship of the lens in the lens barrel, the SEM or the like has a structure in which the final stage condenser lens CLn cannot be used in the reduction system. The reduction of the beam is mainly performed by the first-stage condenser lens CL1 and the like and the objective lens OL. The fact that the last-stage condenser lens CLn was not actually used as a reduction system is the reason why Koehler illumination could not be a parallel beam generation system in an electron microscope.

  4 to 7 are diagrams showing simulation results of electron trajectories when the magnification of the front lens (final condenser lens CLn) is changed. Here, in the above-described two-stage irradiation system, a parallel beam is realized behind the rear-stage lens (objective lens OL). In each figure, the electron trajectory crosses in the gap region where the positive and negative magnetic poles face each other for the front lens (CLn) and the rear lens (OL). This is because a magnetic field type electron lens is used as the lens, and thus the electron beam rotates in the lens. Moreover, as shown in each figure, in these cases, a trajectory in which the on-axis beam and the off-axis beam are separated from each other is drawn.

  Here, the simulation conditions of FIGS. 4 to 7 are as follows. Here, a simulation was performed on the two-stage irradiation system shown in FIG. The calculation area 72 has a range of 125 mm from −15 mm to 110 mm with respect to the x-axis direction (direction of the optical axis 70) and 70 mm from 0 mm to 70 mm with respect to the y-axis direction (direction perpendicular to the optical axis 70). When this is made to correspond to the scale (graph paper) shown in FIGS. 4 to 7, the size of one scale in the horizontal direction (x-axis direction) of the scale is 10 mm, and one scale in the vertical direction (y-axis direction). The dimension is 5 μm. The acceleration voltage was set to 30 kV. Further, the excitation conditions of the front lens (final condenser lens CLn) are increased in the order from FIG. 4 to FIG. The excitation condition for each lens is set by an ampere turn (AT). The excitation condition (ampere turn) of the rear lens (OL) is matched to the Koehler illumination condition. That is, in this simulation, when the excitation condition of the front lens (final condenser lens CLn) is changed, the excitation condition of the rear lens (OL) is readjusted so as to maintain the Koehler illumination condition.

  FIG. 4 shows a case where the magnification (magnification) is increased by about 2.5 times with the previous lens (CLn) by increasing the magnification by weakening the excitation condition.

  Next, the case where the front lens (CLn) is used in the reduction mode will be described. This is the case with FIGS.

  First, FIG. 5 shows a case where the magnification is about 0.75 times. In this case, it seems that a parallel beam is formed by the lens (OL) at the rear stage. However, because of the reduction by the front lens (CLn), the angle of the beam is large, and for this reason, the beam entering the rear lens (OL) is large.

  FIG. 6 shows a case where the reduction ratio of the lens (CLn) at the previous stage is further increased and the magnification is made smaller than 0.5 times. A large reduction ratio of the front lens (CLn) means that the distance between the crossover position and the image plane of the front lens (CLn) is small, so that the off-axis beam also satisfies the conditions of Koehler illumination. To do.

Actually, when the beam angle on the sample is compared between the case of FIG. 5 and the case of FIG. 6, in the case of FIG. 5, the tilt angle of the beam is about 10 ⁻⁴ rad to 10 ⁻⁵ rad. In the case of FIG. 6, the uniformity of about 10 ⁻⁵ rad to 10 ⁻⁶ rad is ensured. The parallelism of the on-axis beam changes gradually by changing the ampere turn of the lens (OL) at the subsequent stage, and the optimum condition can be found by performing a number of repeated calculations to achieve high parallelism. be able to. On the other hand, no matter how fine the adjustment is made, the parallelism of the off-axis beam does not change much. That is, if the crossover of the front lens (CLn) is originally away from the front focal plane, a parallel beam cannot be obtained no matter how exactly the image plane is aligned with the front focal plane, but the reduction ratio is increased. If both are close to each other, a highly parallel beam can be easily realized. For example, in FIG. 6, the ampere turn of the front lens (CLn) is 1200, and the ampere turn of the rear lens (OL) is 400.

  FIG. 7 shows a case where the reduction ratio of the lens (CLn) at the previous stage is further increased. In this case, as a result of bringing the rear lens (OL) closer to the front lens (CLn) than in the case of FIG. 6, the size of the beam irradiation region was reduced to about ¼. In this case, the ampere turn of the front lens (CLn) is 1016, for example, and the ampere turn of the rear lens (OL) is set so as to meet the Koehler illumination conditions.

  FIG. 8 is a diagram illustrating the trajectories of the on-axis beam and the off-axis beam when the magnification of the final condenser lens (CLn) is changed.

  The irradiation lens system of FIG. 8 corresponds to the three-stage irradiation lens system (CL1 + CL2 + OL) shown in FIG. FIG. 8 shows the on-axis beam and off-axis beam when the position of the image plane of the condenser lens (CL2) 34 at the final stage (second stage) is changed in three ways in this three-stage irradiation lens system (CL1 + CL2 + OL). Shows the orbit. The on-axis beam is indicated by a solid line, and the off-axis beam is indicated by a broken line. As shown in FIG. 8, the axial beam is a parallel beam in any case because the objective lens (OL) 40 satisfies the conditions of Koehler illumination. On the other hand, regarding the off-axis beam, depending on the excitation condition (that is, magnification) of the condenser lens (CL2) 34 at the final stage, there are cases where the off-axis beam becomes a parallel beam and a non-parallel beam. This continuously changes with respect to the excitation condition of the condenser lens (CL2) 34.

  As described above, the trajectories in the three cases shown in FIGS. 8A to 8C are the trajectories of the on-axis beam and the off-axis beam in the three-stage irradiation lens system (CL1 + CL2 + OL), and the final stage condenser lens. (CL2) 34 is drawn by changing the focal position (that is, magnification). In the simulations of FIGS. 4 to 7, since the actual trajectory including aberration and image rotation due to a magnetic field has been seen, it is possible to grasp why or how the off-axis beam is collimated. was difficult. Here, by showing the principle diagram, the numerical display is also used to show under what conditions the parallel beam is realized.

  Looking at the trajectories shown in FIGS. 8A to 8C, all the axial beams are parallel beams. That is, the conditions for Kohler illumination are satisfied.

  In order for the on-axis beam to be parallel, it is necessary to satisfy the conditions of Koehler illumination with respect to the objective lens (OL) 40. Therefore, the condition for making the off-axis beam also parallel is the final stage condenser lens (CL2). ) 34 is further added with other conditions. When viewing the trajectories shown in FIGS. 8A to 8C, the trajectory shown in FIG. 8A has parallel off-axis beams, but FIG. 8B and FIG. Each orbit shown in (C) no longer has parallelism. Further, the inclination angle of the off-axis beam is larger in the case of FIG. 8C than in the case of FIG. As described above, FIGS. 8A to 8C are different in the focal position of the condenser lens (CL2) 34 at the final stage. In particular, the difference between FIG. 8A, FIG. 8B, and FIG. 8C is that the focal plane (that is, the image plane) of the last condenser lens (CL2) 34 is the same as FIG. In FIG. 8 (B) and FIG. 8 (C), the object lens (OL) 40 is approached.

  FIG. 9 is a graph showing the relationship between the focal position of the objective lens and the beam irradiation angle of the off-axis beam onto the sample.

Specifically, FIG. 9 shows the relationship between the focal position of the objective lens (OL) (mm) (see _{P 3} in FIG. 2), the beam irradiation angle onto the sample off-axis beam of radius 1 [mu] m (mrad) Is shown. As shown in FIGS. 10 and 11, in this simulation, simulation calculation was performed for eight cases from case a to case h. Here, only five cases excluding cases a, g, and h are used. Show. For example, case b corresponds to FIG. 7, case d corresponds to FIG. 6, and case f corresponds to FIG.

For example, in the case f, a parallelism of 10 ⁻⁵ rad is realized except for one point. The case shown here is an ideal case that does not take into account the aberration of the lens and so on, and is a fairly good value. In the actual beam, cases b, d, and f are shown in order in FIGS. 7, 6, and 5, respectively. However, in the case of case a, case b, and case c, they appear as parallel beams. Met. In particular, in case a, it seems that the parallelism is 10 ⁻⁶ rad or less. The reason why the parallel beam can be obtained is that the convergence position (that is, the image plane) of the condenser lens (CL2) 34 at the final stage is on the left side of the focal position Z = 100 mm of the objective lens (OL) 40. Is the case. That is, when the focus of the objective lens (OL) 40 is located on the beam reduction side than the last stage of the condenser lens (CL2) 34 and the objective lens (OL) 40 midpoint (see _{P 5} in FIG. 2), The off-axis beam is also parallel.

  This will be described with reference to FIG. 8. The off-axis beam is collimated under the condition that the off-axis beam trajectory indicated by the broken line almost overlaps the on-axis beam trajectory indicated by the solid line. This is quite obvious, as can be seen from FIG. If the on-axis beam and the off-axis beam have the same trajectory, the on-axis beam is originally set to a parallel beam condition, and thus the off-axis beam is also collimated.

  In other words, when the reduction lens system is used, the crossover surface approaches the image surface (the surface on which the beam is actually formed). Therefore, the closer the crossover plane and the image plane are, the closer both the on-axis beam and the off-axis beam trajectories are to the same trajectory. Since the on-axis beam satisfies the conditions of Koehler illumination, if the off-axis beam trajectory is brought closer to the on-axis beam trajectory by adjusting the excitation condition of the condenser lens CLn at the final stage, the off-axis beam also Parallelized.

  FIG. 10 is a graph showing the relationship between the focal position of the objective lens and the magnification of the condenser lens.

  Specifically, in the graph of FIG. 10, the left vertical axis indicates the composite magnification of the two-stage condenser lens (CL1 + CL2) 30, and the right vertical axis indicates the second-stage second condenser lens (CL2) 34. The single magnification is shown. Each magnification has a linear relationship with the image plane position of the second condenser lens (CL2) 34 (that is, the focal position of the objective lens (OL) 40). For example, when the single magnification M (CL1) of the first condenser lens (CL1) 32 is set to 0.2 times, when the combined magnification M (CL1 + CL2) is 0.2 times, the second condenser lens (CL2) 34 The single magnification M (CL2) is 1 time. As shown in FIG. 10, the case where the single magnification M (CL2) of the second condenser lens (CL2) 34 is 1 is the case a, case b, and case c.

  The disadvantage of this method is that the beam size on the sample increases, so that if the beam is cut by a stop to make a beam in a very small area, the illumination becomes dark. This is shown in FIG. 11 below.

  FIG. 11 is a graph showing the relationship between the focal position of the objective lens and the beam radius on the sample.

  Specifically, FIG. 11 shows an on-axis beam (solid line) and an off-axis beam (radius 1 μm) with respect to the image plane position of the last condenser lens (CL2) 34, that is, the focal position of the objective lens (OL) 40. The radius on the sample is indicated by a broken line. When collimation of off-axis beams is realized (see case a, case b, and case c), it can be seen that the increase in beam size is particularly significant.

  As a contrivance for suppressing such an increase in beam size, a method of shortening the distance between the last-stage condenser lens CLn and the objective lens OL can be considered. This can be understood as a method of narrowing the beam irradiation region while ensuring the parallelism of the beam.

  In general, as the magnification of the microscope increases, the area of the observation region decreases. However, since the area of the detector is the same, in order to increase the amount of light striking the detector to a certain extent, a stronger beam must be used as the magnification increases. Conventionally, it has been necessary to endure a dark beam because the beam cannot be narrowed by parallel beam irradiation. However, as a device for eliminating the spread of the beam, for example, as described above, by shortening the distance between the condenser lens CLn at the final stage and the objective lens OL, a bright beam can be secured even by parallel irradiation.

  Specifically, the distance between the condenser lens CLn at the final stage and the objective lens OL is usually set to be between the lenses in a normal electron microscope because a deflection system, an astigmatism correction system, a scanning system, and the like are placed between the two lenses. A large distance is taken, not just the distance by stacking. In order to reduce the irradiation area and prevent a decrease in luminance, it is necessary to devise a method for making the distance between the last-stage condenser lens CLn and the objective lens OL as close as possible.

For example, the first contrivance for this is to move the deflection system, astigmatism correction, scan system, etc. not to the rear of the last condenser lens CLn, but to the rear of the condenser lens CLn-1 in the previous stage. 12, the distance between the last condenser lens (CL2) 34 and the objective lens (OL) 40 is physically shortened (d ₁ > d ₂ > d ₃ ). In this case, it is necessary to adjust the excitation condition of the objective lens (OL) 40 so that the focal length of the objective lens (OL) 40 becomes shorter as the distance between the lenses 34 and 40 becomes shorter. That is, in this case, in the objective lens (OL) 40, the excitation condition (driving condition) is set so that the distance between the last stage condenser lens (CL2) 34 and the objective lens (OL) 40 is shortened.

  In addition, the second device is to give the electron lens a structure in which the position of each pole piece approaches between the last-stage condenser lens CLn and the objective lens OL. Specifically, as shown in FIG. 13, the pole piece 64a of the last stage condenser lens (CL2) 34a and the pole piece 64b of the objective lens (OL) 40a are each at a position where the mutual distance becomes short, that is, They are provided at positions facing each other (see FIG. 3 for comparison).

  The two exemplified methods may be used in combination, or only one of them may be used. As described above, even if the distance between the two lenses (the last stage condenser lens CLn and the objective lens OL) is shortened, there is no particular adverse effect on the parallelism of the off-axis beam.

  As described above, in the magnetic type electron lens, electrons move in a spiral shape, so that the image rotates. In this case, since the angle is given by the rotation of the image, even if a parallel beam is made, it is inclined again by the angle generated by the rotation of the image. Therefore, a device for preventing this is necessary.

  The first device for preventing image rotation is to adjust the total of all ampere turns so that the total image rotation angle by the magnetic field becomes zero for all the electron lenses.

  The second device is to configure each electron lens with a double gap lens. That is, for each electron lens, two lenses having opposite excitation directions are combined so that image rotation is canceled.

  FIG. 14 is a diagram for explaining the latter device, that is, a case where a double gap lens is used in order to eliminate image rotation.

  In order to eliminate the rotation of the image, a double gap lens may be used for each condenser lens (CL1, CL2) and objective lens (OL). FIG. 14A shows a final-stage condenser lens (CL2) 34b and objective lens (OL) 40b each using a double gap lens. As shown in FIG. 14A, the double gap lens is a coil in which a first coil 80 and a second coil 82 are paired. By applying reverse currents to these coils 80 and 82, the leakage magnetic fields generated in the first gap 84 and the second gap 86 have opposite polarities, and the leakage magnetic field along the optical axis 70 The image rotation effect acting on the electron beam passing therethrough cancels out. The simulation result of the electron trajectory corresponding to the configuration of FIG. 14A is as shown in FIG. In the simulation of FIG. 14B, the gap between the pole pieces corresponding to the first coil 80 and the second coil 82 was calculated as 6 mm.

  In the present embodiment, the case where the condenser lenses (CL1, CL2) 32 and 34 and the objective lens (OL) 40 are each configured with an electron lens (magnetic type electron lens) has been described as an example. It is not limited to. Instead of the magnetic type electron lens, it is also possible to use an electron lens (electrostatic type electron lens) that focuses an electron beam by an electrostatic field.

  Hereinafter, a specific application example to the apparatus will be described. It should be noted that the application example described below is merely an example and is not limited thereto.

  FIG. 15 is a schematic diagram illustrating an example of an optical system of a diffraction imaging apparatus as a first application example of the electron beam irradiation apparatus according to the present embodiment. This application example is a case where a sample is irradiated with a beam using a condenser lens and an objective lens. In FIG. 15, the same components as those in the electron beam irradiation apparatus 10 shown in FIG.

  A diffraction imaging apparatus 100 shown in FIG. 15 includes, as an optical system 102, an electron gun 20, a two-stage condenser lens 30 (a first condenser lens (CL1) 32 and a second condenser lens (CL2) 34), an objective lens 40, In addition to the sample 50, a first deflector 110, an astigmatism corrector 120, and a second deflector 130 are sequentially provided between the second condenser lens (CL2) 34) and the objective lens 40. The first deflector 110 and the second deflector 130 constitute a two-stage deflection system, and the electron beam is deflected by the first deflector 110 at the first stage and deflected by the second deflector 130 at the second stage. Turn the beam back. The astigmatism corrector 120 is a device for correcting astigmatism of the objective lens (OL) 40. As described above, the electron beam that has passed through the objective lens 40 becomes a parallel beam and irradiates the sample 50 in parallel. The diffraction pattern from the sample 50 is detected by the detector 140.

  FIG. 16 is a ray diagram of the diffraction imaging apparatus of FIG.

  As shown in FIG. 16, the electron beam generated by the electron gun 20 is irradiated with the irradiation lens system 12 shown in FIG. 2, that is, the first condenser lens (CL1) 32, the second condenser lens (CL2) 34, and the objective lens ( OL) 40 generates a parallel electron beam and irradiates the sample 50.

  FIG. 17 is a schematic diagram illustrating an example of an optical system of a Lorentz electron microscope as a second application example of the electron beam irradiation apparatus according to the present embodiment. In this application example, the beam is irradiated onto the sample using only the condenser lens without using the objective lens. The same components as those of the electron beam irradiation apparatus 10 shown in FIG. 2 and the diffraction imaging apparatus 100 shown in FIG. 15 are denoted by the same reference numerals, and the description thereof is omitted.

  In the diffractive imaging apparatus 100 shown in FIG. 15, in order to realize parallel irradiation onto the sample 50, a parallel beam is formed by the second condenser lens (CL2) 34 and the objective lens (OL) 40. The Lorentz shown in FIG. In the electron microscope 200, in order to realize parallel irradiation onto the sample 50, the first condenser lens (CL1) 32 and the second condenser lens (CL2) 34 are configured to form a parallel beam.

  The Lorentz electron microscope 200 includes, as an optical system 202, in order from the electron gun 20, a two-stage condenser lens 30 (a first condenser lens (CL1) 32 and a second condenser lens (CL2) 34), a first deflector 110, It has an astigmatism corrector 120, a second deflector 130, a sample 50, and an objective lens 40. Furthermore, a first intermediate lens (IL1) 210 and a second intermediate lens are arranged between the objective lens 40 and the detector 140 in this order. (IL2) 220 and a projection lens (PL) 230. Here, in the sense that no objective lens is used, the irradiation lens system 204 is configured only by the two-stage condenser lens 30 (first condenser lens (CL1) 32 and second condenser lens (CL2) 34). . The intermediate lenses (IL1, IL2) 210 and 220 are lenses between the objective lens and the projection lens, and are configured in two stages here. The projection lens (PL) 230 is the final lens of the imaging lens system, and further enlarges the image enlarged by the intermediate lens and forms an image on the detector 140. In this case, the electron beam that has passed through the second condenser lens (CL2) 34 becomes a parallel beam and is irradiated onto the sample 50 in parallel.

  18 is a ray diagram of the Lorentz electron microscope of FIG.

  As shown in FIG. 18, the electron beam generated by the electron gun 20 becomes a parallel electron beam by the first condenser lens (CL1) 32 and the second condenser lens (CL2) 34 and is irradiated onto the sample 50.

  Thus, according to the present embodiment, a plurality of lenses (for example, a combination of a two-stage condenser lens 30 and an objective lens 40 or only a two-stage condenser lens 30) constituting an irradiation lens system. Among them, the magnification of the lens (the second condenser lens 34 or the first condenser lens 32 in the above example) immediately before the last stage lens (the objective lens 40 or the second condenser lens 34 in the above example) is set. Since it is set to 1x or less and the image plane thereof is arranged so as to coincide with the front focal plane of the last-stage lens, there is no need to add any new device other than the lens shape and relative positional relationship. Moreover, even when the light source size is large, the parallelism of the beam can be increased.

  The electron beam irradiation apparatus according to the present invention increases the parallelism of the beam without adding any new apparatus other than the lens shape and relative positional relationship, and even when the light source size is large. It is useful as an electron beam irradiation apparatus capable of

DESCRIPTION OF SYMBOLS 10 Electron beam irradiation apparatus 12, 204 Irradiation lens system 20 Electron gun 22 Crossover 30 Condenser lens 32 1st condenser lens (CL1)
34, 34a, 34b Second condenser lens (CL2)
40, 40a, 40b Objective lens (OL)
50 Sample 60 Coil 62 Yoke 64, 64a, 64b Pole piece 70 Optical axis 80 First coil 82 Second coil 84 First gap 86 Second gap 100 Diffraction imaging apparatus 102, 202 Optical system 110 First deflector 120 Astigmatism correction Device 130 Second deflector 140 Detector 200 Lorentz electron microscope 210 First intermediate lens (IL1)
220 Second intermediate lens (IL2)
230 Projection lens (PL)

Claims (12)

An electron beam source for generating an electron beam;
An irradiation lens system that collects the generated electron beam and applies it to the sample, and
The irradiation lens system includes a plurality of lenses,
Among the plurality of lenses, the lens immediately preceding the last-stage lens is set so that the magnification is set to 1 or less and its image plane coincides with the front focal plane of the last-stage lens. And
The distance between the crossover position of the electron beam source and the image plane of the previous lens is d ₁ , and the distance between the crossover position of the electron beam source and the main plane of the previous lens is d ₂ , when the distance between the main plane of the last stage of the lens and the crossover position of the electron beam source was a d _3, further satisfying the following relationship,
d ₁ ≦ d ₂ + (d ₃ −d ₂ ) / 2
Electron beam irradiation device. The irradiation lens system is
One or more condenser lenses that collect the generated electron beam;
An objective lens that applies the electron beam collected by the condenser lens to the sample, and
Of the one or more condenser lenses, the last stage condenser lens is arranged such that the magnification is set to 1 or less and the image plane thereof coincides with the front focal plane of the objective lens.
The electron beam irradiation apparatus according to claim 1. The irradiation lens system is
Having a plurality of condenser lenses, irradiating the sample with an electron beam using the plurality of condenser lenses without using an objective lens,
Among the plurality of condenser lenses, the condenser lens that is one stage before the last stage condenser lens is set to have a magnification of 1 or less, and its image plane coincides with the front focal plane of the last stage condenser lens. Located in the
The electron beam irradiation apparatus according to claim 1. The preceding lens is
Excitation conditions are adjusted to satisfy the above relationship ,
The electron beam irradiation apparatus according to claim 1. The objective lens is a magnetic field-free electron lens that does not apply a magnetic field to the sample.
The electron beam irradiation apparatus according to claim 2. Each of the plurality of lenses is an electrostatic electron lens.
The electron beam irradiation apparatus according to claim 1. Each of the plurality of lenses is a magnetic field type electron lens, and all the lenses are adjusted to have a total ampere turn so that a total image rotation angle by the magnetic field becomes zero, or each lens is a double gap lens. It is configured,
The electron beam irradiation apparatus according to claim 1. The drive condition is set so that the objective lens is disposed close to the last-stage condenser lens ,
The electron beam irradiation apparatus according to claim 2. Each of the last-stage condenser lens and the objective lens is a magnetic field type electron lens,
The pole piece of the last-stage condenser lens and the pole piece of the objective lens are provided at positions facing each other ,
The electron beam irradiation apparatus according to claim 2. Electron microscope having an electron beam irradiation apparatus as claimed in any one of claims 9. At least one of a deflector that deflects and scans an electron beam and an astigmatism corrector that corrects astigmatism is disposed between the last-stage condenser lens and the condenser lens that is one stage before it.
The electron microscope according to claim 10 .   A Lorentz electron microscope comprising the electron beam irradiation apparatus according to claim 3.

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