JP3692011B2 - Magnetic field type energy filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic energy filter that has a plurality of magnetic field regions and deflects the trajectory of an electron beam from an entrance window to an exit slit.
[0002]
[Prior art]
4 is a diagram showing a configuration example of an electron microscope in which an omega energy filter is incorporated in an electron optical system, FIG. 5 is a diagram for explaining a configuration of an A type omega energy filter, and FIG. 6 is a diagram of a B type omega energy filter. FIG. 7 is a diagram for explaining the configuration, FIG. 7 is a diagram for explaining the basic trajectory of the A type omega energy filter, and FIG. 8 is a diagram for explaining the basic trajectory of the B type omega energy filter.
[0003]
As an energy filter used in connection with an electron microscope, an in-column type energy filter such as an omega filter or a Casttain-Henry type filter is used in the electron microscope while keeping the lens barrel straight. It is actively used to incorporate it into In an electron microscope incorporating an omega energy filter in an electron optical system, as shown in FIG. 4, an electron beam generated by an electron gun 11 is irradiated onto a sample 14 through a condenser lens 12, and an objective lens 13, an intermediate lens 15, and an incident window 16 are irradiated. The observation image of the sample is projected onto the fluorescent plate 20 through the omega energy filter 17, the exit slit 18, and the projection lens 19. This omega energy filter allows an incident beam to pass through four magnets M1, M2, M3, and M4 (beam rotation radii R1, R2, R3, and R4) arranged in an Ω-shaped orbit while sequentially deflecting the beam. And the outgoing beam are arranged on the same straight line, and FIG. 5 and FIG. 6 show two examples of the shape of the magnet pole piece and the electron trajectory. Here, a straight line in which the incident beam and the outgoing beam are arranged on the same straight line or a straight line passing through the incident window and the outgoing slit is called a “linear axis” and is deflected by the magnetic field of the filter as shown in FIGS. The center trajectory of the received beam is referred to as an “optical axis indicating the center trajectory of the filter”.
[0004]
The apparatus in which the omega energy filter is inserted in the middle or rear of the imaging lens system of the transmission electron microscope is used as an apparatus for electron spectroscopic imaging (ESI). In devices such as omega energy filters and alpha-type energy filters, the optical axis of the incident beam and the optical axis of the outgoing beam are in a straight line and have a plurality of magnetic field regions, and deflect the trajectory of the electron beam from the entrance window to the exit slit. Therefore, it is used by being inserted in the middle of the imaging lens system and is called an in-column type ESI device. On the other hand, a filter combining a single sector magnet and a multipole correction device is installed behind the microscope tube because the optical axis of the outgoing beam changes in the direction of about 90 ° from the incident beam. It is called a column type filter.
[0005]
The omega energy filter is a typical in-column type filter, and this filter is a static type of in-column type filter that is a combination of a magnetic prism, an electrostatic mirror, and a magnetic prism, originally called a caster Henry filter. The electromirror is replaced with a magnetic prism and all deflecting elements start with a magnetic field. Therefore, the filter developed in France in the 1970s consisted of three magnetic fields, but research on the aberration theory of the filter was later conducted in Germany, and four magnets were used rather than three magnets. Was found to be advantageous, and subsequent work was studied in a system using four magnets.
A sector magnet having a uniform field has a beam converging effect in a direction x parallel to the magnetic pole surface, that is, a direction in which energy dispersion occurs, but has no converging effect in the magnetic field direction y. Therefore, in the case of the omega energy filter, the magnetic pole end face is inclined, and the convergence in the magnetic field direction is obtained by the quadrupole lens action produced by the end face inclination. The two examples shown in FIGS. 5 and 6 are designed under different optical setting conditions, and image formation is performed three times in both the parallel direction x and the magnetic field direction y to the magnetic pole surface shown in FIG. A type that performs image formation three times in the parallel direction x and two times in the magnetic field direction y is called the B type. The difference in the basic optical system is apparent in the A-type orbit diagram shown in FIG. 7 and the B-type orbit diagram shown in FIG. 8 in which the optical axis indicating the center orbit of the filter is straightened.
[0006]
Here, both the trajectories xα and yβ are trajectories of an electron beam that finally form a microscopic image to be connected to the fluorescent screen. On the other hand, both the trajectories xγ and yδ are focused on the entrance window surface of the filter by the front lens, and after passing through the filter, focus on the exit slit surface. When the electron beam reaches the exit slit surface, the electron beam is sufficiently dispersed according to its energy. By the exit slit, only a beam having a desired energy range is selected as the electron beam. Note that the image on the fluorescent plate is formed by a beam in the energy range that has passed through the exit slit. However, the dispersion must be extinguished on the image plane because it causes blurring if the dispersion remains. This is called an achromatic condition. Further, the omega energy filter is characterized in that the aperture aberration and distortion aberration on the image plane are eliminated by taking a center plane symmetrical trajectory.
[0007]
The omega energy filter uses a plane between the second magnet M2 and the third magnet M3 as a symmetry plane (center plane) in order to reduce some of the secondary aberrations to zero and to reduce the remaining aberrations. The beam trajectory is designed to be symmetric. That is, when the distance from the image plane to the exit slit plane is LL, the image plane of the incident beam is adjusted so as to be located at a distance LL from the incident window plane. Under such conditions, the difference between the A type and the B type is that, in the orbit in the y direction (magnetic field direction), the A type is yβ = 0 and yδ ′ = 0 on the symmetry plane as shown in FIG. On the other hand, the B type has yβ ′ = 0 and yδ = 0 on the symmetry plane as shown in FIG. Here, “′” is the derivative with respect to z, that is, the inclination of the orbit. The x trajectory is the same condition for both beams, and xα = 0 and xγ ′ = 0 on the symmetry plane for both types.
[0008]
When such an initial condition is selected, in the A type, the xγ trajectory is focused three times as shown in FIG. 7, whereas in the B type, the xδ trajectory is focused three times. However, in the B type, as shown in FIG. Is focused three times, but yδ is focused only twice. That is, the image is turned over. It has long been known that there are two types of omega energy filters.
[0009]
[Problems to be solved by the invention]
In the case of an electron microscope, the energy filter is used as a monochromator for limiting the energy of the electron source to produce a highly monochromatic beam, and electron energy loss spectroscopy (EELS) for measuring the energy loss produced by the sample. In addition, there is use as energy filtered transmitted electron microscopy (EFTEM) in which an image is formed only with zero-loss electrons except for electrons with lost energy, or an image is formed only with lost electrons. The most sought after property for these applications is to have a large dispersion.
[0010]
The best known method for creating large variances is to perform retarding. Since the energy filter usually has a dispersion of about 1/1000 or 1/10000 with respect to the energy of the incident electrons, the energy resolution that can be observed is improved by reducing the energy of the incident electrons. However, for this purpose, the energy filter must be arranged in a field to which a high voltage is applied, so that there is a drawback that the apparatus becomes complicated or large.
[0011]
[Means for Solving the Problems]
The present invention solves the above-mentioned problems, and makes it possible to lengthen the beam path, increase the total absolute value of the beam deflection angle, and realize a compact shape.
[0012]
Therefore, the present invention provides a magnetic field type energy filter that has a plurality of magnetic field regions and deflects the trajectory of the electron beam from the entrance window to the exit slit, and has at least four magnetic field regions. Has a rotationally symmetric axis at the middle position of the magnetic field region, and the polarity of the magnetic field is reversed at the boundary between the second and third magnetic field regions, and the magnetic field of the first and second magnetic field regions The polarity is reversed, the sum of the absolute values of the beam deflection angles in each magnetic field region exceeds 540 °, and there are deflecting magnets on both the left and right sides of the linear axis, and the absolute value of the beam deflection angle in each magnetic field region is The total is over 540 ° or approximately 720 °.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a magnetic energy filter according to the present invention, and FIG. 2 is a diagram for explaining a basic trajectory based on a simulation result of the magnetic energy filter according to the present invention.
[0014]
FIG. 2A shows the trajectory of an electron beam that forms a microscopic image projected onto the entrance window surface. The electron beam that forms a microscope image on the entrance window surface forms a crossover four times in total, and then forms a microscope image again on the exit slit surface. FIG. 2B shows the trajectories of specific (two) energy electron beams of the electron beam that have passed through the entrance window. The electron beam that has passed through the entrance window is focused on the third time, and then focused on the exit slit surface for the fourth time. When the energy of the electron beam is different, the electron beam draws different trajectories and focuses at different positions on the exit slit surface. Therefore, only the electron beam having a desired energy can be selected by the exit slit. FIG. 2C shows a trajectory xα in a direction parallel to the magnetic pole surface of the trajectory of the electron beam forming the microscopic image. FIG. 2D shows a trajectory yβ in the magnetic field direction of the trajectory of the electron beam forming the microscopic image. FIG. 2 (e) shows the trajectory xγ in the direction parallel to the magnetic pole surface of the trajectory of the electron beam of the desired energy and the magnitude of dispersion (xχ) along the optical axis. FIG. 2 (f) shows the trajectory yδ in the magnetic field direction of the trajectory of the electron beam having the desired energy.
[0015]
In FIG. 1, M1 to M4 are magnets, I is an entrance window, and S is an exit slit. The operation in FIG. 1 will be described below along the trajectory of the electron beam. The electron beam enters along the direction of the optical axis indicated by the arrow in the figure, and after crossing over (focusing on) the entrance window I surface, it enters the filter. The electron beam that has passed through the entrance window I enters the magnet M1 having a deflection angle of φ1, is deflected by φ1 (depicted as clockwise deflection in the figure), and exits to form the first focus by the filter. . Next, the electron beam enters a magnet M2 having a deflection angle of φ2, is deflected by φ2 in the reverse direction (counterclockwise in the figure), and exits. The electron beam emitted from the magnet M2 passes through a point O that intersects the linear axis and the electron beam trajectory. Then, the second focus is set near this point O. The electron beam that has passed through the point O is incident on the magnet M3 having a deflection angle of φ3 on the opposite side of the linear axis, is deflected by φ3 in the clockwise direction, and is emitted for the third focus. Here, φ3 = −φ2. Next, the electron beam emitted from the magnet M3 enters the magnet M4 having a deflection angle of φ4, and is deflected by φ4 in the counterclockwise direction before being emitted. Here, φ4 = −φ1. The electron beam emitted from the magnet M4 reaches the exit slit and is focused on the fourth time on the exit slit surface. The electron beam that has reached the exit slit after focusing for the fourth time is in a sufficiently dispersed state due to the difference in energy. Therefore, the electron beam is emitted from the exit slit, and the electron beam having only desired energy passes through the exit slit and exits the filter.
[0016]
Here, the point O at the intermediate position between the magnet M2 and the magnet M3 is referred to as an intermediate point. The axis that passes through this point and is perpendicular to the paper surface is the two-fold rotational symmetry axis (or two-fold rotational axis) of the filter. That is, this filter is rotationally symmetric twice about the axis passing through the point O.
[0017]
Thus, in the magnetic field type energy filter according to the present invention, the magnets are arranged on both sides of the linear axis, and they are rotationally symmetric twice, so that conventional filters such as omega filters and alpha filters are linear. Compared with a magnet only on one side of the shaft, the beam path is lengthened without increasing the distance between the inlet and outlet of the filter, that is, the distance D between the entrance window I and the exit slit S. The sum of the absolute values of the corners can be increased.
[0018]
Therefore, according to the magnetic field type energy filter according to the present invention as described above, the shape is more compact than the conventional omega filter. In addition, in the case of the configuration shown in FIG. 1, if the beam deflection angle of the magnet is 110 °, −250 °, 250 °, and −110 ° in order from the incident side, the total absolute value of the deflection angle is 720 °. Become. This beam deflection angle is increased to twice that of the alpha filter. Even in the case of the conventional omega filter, even if the practical deflection angle limit is set to 125 ° with four magnets, and 125 °, −125 °, −125 °, and 125 °, the total absolute value of the deflection angle is Since it is 500 °, it is increased by about 1.4 times.
[0019]
When the conventional energy filters proposed so far are composed of four magnetic field regions of the magnetic fields 1 to 4, the magnetic field polarities of the magnetic field 2 and the magnetic field 3 sandwiching the symmetry plane are all the same. In the magnetic field type energy filter newly proposed in the present invention (referred to as S filter), the polarities of the magnetic field 2 and the magnetic field 3 are reversed with respect to the rotational symmetry axis.
[0020]
1 and 2 show an example in which the sum of the absolute values of the deflection angles is 720 °, but in the S filter in which the polarities of the magnetic fields 1 and 2 are reversed, the absolute value of the deflection angle. If the sum of the angles exceeds 540 °, the beam path is lengthened without increasing the distance between the entrance and exit of the filter, that is, the distance D between the entrance window I and the exit slit S, and the sum of absolute values of the beam deflection angles is increased. Can achieve the purpose of enlarging.
[0021]
Another important characteristic is the number of times the xγ orbit intersects the optical axis. In general, it may be considered that the greater the number of times the xγ trajectory intersects the optical axis indicating the center trajectory of the filter, the greater the dispersion. The number of times the xγ trajectory of the conventional omega filter intersects the optical axis indicating the center trajectory of the filter is 3 for both the A type in FIG. 5 and the B type in FIG. 6 as indicated by the arrows in FIG. 7 and FIG. Times. On the other hand, the filter of the present invention is four times as indicated by an arrow in FIG.
[0022]
FIG. 3 is a diagram showing another embodiment of the magnetic field type energy filter according to the present invention, wherein M5 to M8 are magnets, I is an entrance window, and S is an exit slit. In FIG. 3, the magnets M5 and M6 on the beam entrance side with the entrance window are set to have the same beam deflection direction, and the magnets M7 and M8 on the exit side with the exit slit are magnets M5 and M5, The magnetic field is opposite to that of M6 and is arranged so as to be twice rotationally symmetric. That is, the beam deflection angles φ7 (= −φ6) and φ8 (= −φ5) of the magnets M7 and M8 are set to the beam deflection angles φ5 and φ6 of the magnets M5 and M6. This is called an 8-character filter. In this 8-shaped filter, the total absolute value of the beam deflection angle is approximately 720 °.
[0023]
Supplementally, it can be said that the magnetic energy filter according to the present invention shown in FIG. 1 is a modification of the omega filter, and the magnetic energy filter according to the present invention shown in FIG. 3 is a modification of the alpha filter.
[0024]
In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible. For example, in the above embodiment, the omega filter or the alpha filter is deformed and the magnets are arranged so as to be rotationally symmetrical twice on both sides of the linear axis, but the omega filter is modified on one side of the linear axis, that is, the S filter is provided. It may be configured by combining different types of magnets on both sides of the linear axis, such as adopting a modified alpha filter on the opposite side of the linear axis, that is, adopting an 8-shaped filter.
[0025]
Moreover, although the said embodiment demonstrated with the structure which consists of four magnets M1-M4 and M5-M8, this invention showed that a magnetic field type energy filter comprised by four magnetic field area | regions. Yes, for example, one or all of the magnets M2 and M3 shown in FIG. 1 and the magnets M6 and M7 shown in FIG. 3 are divided into two parts (for example, M2 is divided into two parts M2-1 and M2-2). Needless to say, it may be. That is, even if a magnet is divided and a drift space without a magnet is inserted between them, it is essentially the same.
[0026]
【The invention's effect】
As is apparent from the above description, according to the present invention, in the magnetic field type energy filter having a plurality of magnetic field regions and deflecting the trajectory of the electron beam from the entrance window surface to the exit slit surface, at least four magnetic field regions. The polarity of the magnetic field is reversed at the rotational symmetry axis located between the second and third magnetic field regions, and the magnetic field polarity is reversed between the first and second magnetic field regions. The sum of the absolute values of the beam deflection angles in each magnetic field region is over 540 °, and there are deflecting magnets on both the left and right sides of the linear axis, and the sum of the absolute values of the beam deflection angles in each magnetic field region is 540 °. Compared to conventional omega energy filters and alpha energy filters, the beam path can be made longer and the total absolute value of the deflection angle can be made larger than the conventional omega energy filter or alpha energy filter. The distance can be shortened to realize a compact energy filter.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of a magnetic energy filter according to the present invention.
FIG. 2 is a diagram for explaining a basic trajectory of a magnetic field type energy filter according to the present invention and a comparison thereof.
FIG. 3 is a diagram showing another embodiment of the magnetic field type energy filter according to the present invention.
FIG. 4 is a diagram illustrating a configuration example of an electron microscope in which an omega energy filter is incorporated in an electron optical system.
FIG. 5 is a diagram for explaining a configuration of an A type omega energy filter.
FIG. 6 is a diagram for explaining a configuration of a B type omega energy filter.
FIG. 7 is a diagram for explaining a basic trajectory of an A type omega energy filter.
FIG. 8 is a diagram for explaining a basic trajectory of a B type omega energy filter.
[Explanation of symbols]
M1 to M4 ... magnet, I ... entrance window, S ... exit slit

Claims (5)

In a magnetic field type energy filter that has a plurality of magnetic fields and deflects the trajectory of an electron beam from an entrance window to an exit slit, it has at least four magnetic field regions, 2 It has a rotational symmetry axis at the middle position between the first and third magnetic field regions, and the polarity of the magnetic field is reversed at the boundary between the second and third magnetic field regions. A magnetic field type energy filter in which the polarity of the magnetic field is reversed between the eye and the second magnetic field region, and the sum of the absolute values of the beam deflection angles of the respective magnetic field regions exceeds 540 ° . The magnetic field type energy filter according to claim 1, wherein the number of the magnetic field regions is four, and the beam deflection angles of the magnetic field regions are 110 °, -250 °, 250 °, and -110 ° in order from the incident side . In the magnetic field energy filter for deflecting the trajectory of the electron beam to the exit slit (exit Slit) from a plurality of magnetic regions (Magnetic fields) to have entrance window (entrance The entrance window), have at least four magnetic field regions, 2 a-th and the axis of rotational symmetry in an intermediate position of the 3 -th field region (rotational symmetry axis), the polarity is inverted magnetic field and between the two eyes and three eyes of the magnetic field region as a boundary, one A magnetic field type energy filter in which the polarity of the magnetic field is the same between the eye and the second magnetic field region . 4. The magnetic field type energy filter according to claim 3, wherein a total of absolute values of beam deflection angles of the respective magnetic field regions is about 720 [deg .]. In the magnetic field type energy filter having a plurality of magnetic field regions and deflecting the trajectory of the electron beam from the entrance window to the exit slit, the magnet has deflection magnets on both left and right sides of a linear axis connecting the entrance window and the exit slit, A magnetic field type energy filter in which the total absolute value of beam deflection angles in each magnetic field region is approximately 720 ° .

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