JP4150568B2 - electronic microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scanning transmission electron microscope that includes an energy analyzer and can simultaneously observe an energy analysis and a dark field image.
[0002]
[Prior art]
Conventionally, a transmission electron microscope (TEM) equipped with an energy analyzer that analyzes the energy of an electron beam has been provided (see, for example, Patent Document 1).
[0003]
[Patent Document 1]
JP 2000-133195 A
[0004]
Such an electron microscope is mainly used for the following three applications. The first is to obtain an energy spectrum of the sample by electron energy loss spectroscopy (EELS) using an energy analyzer. The second is a scanning transmission electron microscope (STEM) system that observes a microscope image obtained by scanning a sample with an electron beam. When this method is used, a spectrum by EELS acquired for each position of a finely focused electron beam for scanning the sample is stored in a memory. After scanning the sample, a microscopic image with a specific energy is formed using data stored in the memory. This is called a spectral imaging method. Third, there is a direct mapping transmission electron microscope (TEM) system. In the case of this method, a microscopic image is formed by an electron beam having a desired energy selected by an energy analyzer.
[0005]
In order to form a microscopic image by the spectral imaging method, a diffraction image by a sample is usually formed on the entrance window surface. This is because when a microscopic image of the sample is formed on the entrance window surface, this image moves in response to the scanning of the electron beam, whereas the diffraction image of the sample is stationary regardless of the scanning of the electron beam, so it is easy to handle. That's why. In such a stationary diffraction image, a desired diffraction spot can be selected by the objective aperture.
[0006]
In the energy analyzer, the electron beam is dispersed according to energy on the exit window surface corresponding to the entrance window surface. Therefore, an electron beam having a desired energy can be selected by providing a slit on the exit window surface. Further, when observing a spectrum by EELS, the optical system at the subsequent stage magnifies and projects the incident image plane onto the recording observation apparatus. In order to increase the energy resolution of the electron beam, it is necessary to reduce the diameter of the electron beam on the entrance window surface and the entrance image plane as much as possible.
On the other hand, conventionally, a high angle annular dark field (HAADF) that forms an electron beam diffracted at a large angle from a sample has been used (for example, see Non-Patent Document 1).
[0007]
[Non-Patent Document 1]
“Artificial bright spots in atomic-resolution high-angle annular dark field STEM images,” T. Yamazaki, M. Kawasaki, K. Watanabe, I. Hashimoto, M. Shiojiri, Journal of Electron Microscopy 50 (6) 517-521 ( 2001)
[0008]
[Problems to be solved by the invention]
FIG. 8 is a diagram illustrating a configuration of an energy filter including an annular detector. In this energy filter, an annular detector 121 having a hole in the center so as to transmit an electron beam is disposed between the incident window surface 111 and the incident image surface 112 along the optical axis L0, and a spectrum is acquired by EELS. At the same time, the electron beam diffracted at a large angle is detected by the annular detector 121.
[0009]
In the illustrated energy analyzer, an electron beam diffracted from a sample at a large angle needs to be detected by the annular detector 121, so that the objective aperture cannot be inserted. Since the diffraction image of the sample formed on the entrance window surface depends on the angle of the electron beam diffracted by the sample, the diameter increases without an objective aperture.
[0010]
FIG. 9 is a diagram for explaining the electron beam emission angle and the electron beam diameter at the entrance window surface.
[0011]
For example, if the emission angle θ11 of the diffracted electron beam detected by the annular detector is 170 mrad, the electron beam diameter D11 at the entrance window surface 111 can be reduced to about 1 μm. In this state, the energy filter can be used with an energy resolution of 1 eV or less. Note that black spots in the figure are diffraction spots.
[0012]
FIG. 10 is a diagram showing the shape of the electron beam on the exit window surface and the exit image plane of the energy analyzer.
[0013]
10 (a) and 10 (b) show the electron beam diameter on the exit window surface and the exit image surface when the electron beam diameter on the entrance window surface is 0.9 μm and the electron beam diameter on the entrance image surface is 9 μm. It is a figure which shows each shape. In addition, the spot in FIG.10 (b) is expanded 200 times.
[0014]
In the exit window shown in FIG. 10 (a), the right electron beam has no energy loss incident at an energy of 200 kV, and the left electron beam has lost energy by 1 eV. As can be seen from the figure, the two beams are sufficiently separated, and it is estimated that an energy spectrum with an energy resolution of 1 eV or less can be obtained. Thus, there is no problem in observing the spectrum by EELS and spectrum imaging using the spectrum.
[0015]
FIG. 11 is a diagram for explaining the diameter of the electron beam on the incident window surface.
[0016]
The electron beam has an emission angle θ11 of 170 mrad, and the electron beam diameter D11 is reduced and projected to 0.9 μm. Here, the size of the annular detector 121 arranged at the subsequent stage of the entrance window surface 111 becomes a problem. That is, when an electron beam having a half angle θ12 of about 10 mrad is transmitted through the inner hole of the annular detector 121 and is incident on the energy analyzer, the diameter D12 of the inner hole of the annular detector 121 is 0.9 μm × (10/170 ) = 53 nm is required. On the other hand, the annular detector 121 may have any outer diameter, but the electron beam to be measured is concentrated to about 1 μm in diameter around the optical axis L0. According to modern nanotechnology, it is not impossible to manufacture an annular detector 121 having a detector having an inner diameter of 53 nm and an outer diameter of about 1 μm as described above, but it is not realistic in view of ease of manufacturing and cost. .
[0017]
In this way, in a scanning transmission electron microscope equipped with an energy analyzer, in order to observe energy analysis and dark field images simultaneously, the electron beam is narrowed down and diffracted at the entrance window surface of the energy analyzer to ensure energy resolution. There was a need to choose light.
[0018]
The present invention has been proposed in view of the above circumstances, and an object thereof is to provide a scanning transmission electron microscope including an energy analyzer and including an annular detector.
[0019]
[Means for Solving the Problems]
In order to solve the above-described problems, an electron microscope according to the present invention is an apparatus for observing a transmission image by scanning a sample with an electron beam, an energy analyzer for analyzing the electron beam scanned on the sample for energy, An annular detector that is arranged in front of the energy analyzer in the traveling direction of the electron beam and detects an electron beam scattered at a large angle by the sample, and has a small aperture of the sample on the incident window surface of the energy analyzer. A spectrum or a microscope image is created using the electron beam analyzed by the energy analyzer.
[0020]
Preferably, the aperture is in the range of 0.01 μm to 10 μm.
[0021]
Preferably, the distance between the annular detector and the incident window surface is at least half of the distance between the incident window surface and the incident image surface.
[0022]
Preferably, a diaphragm for limiting an incident angle or diameter of the electron beam is disposed between the annular detector and the energy analyzer along the electron beam.
[0023]
Preferably, the aperture of the diaphragm is in the range of 1 μm to 10 μm.
[0024]
Preferably, the sample is scanned, a spectrum analyzed by the energy analyzer is stored in a memory for each position in the sample, and the spectrum stored in the memory is signal-processed to obtain a spectrum.
[0025]
Preferably, the electron microscope according to the present invention is an electron microscope that scans a sample with an electron beam and observes a transmission image, an energy analyzer that analyzes the electron beam scanned on the sample for energy, and an incident image plane of the energy analyzer. An annular detector for detecting an electron beam scattered at a large angle by the sample, and projecting a microscopic image of the sample with a small aperture on the entrance window surface of the energy analyzer, A spectrum or microscopic image is created using the electron beam analyzed by the analyzer.
[0026]
Preferably, the aperture is in the range of 0.01 μm to 10 μm.
[0027]
Preferably, a diaphragm for limiting the incident angle or diameter of the electron beam is disposed in the vicinity of the incident image plane in the energy analyzer along the electron beam.
[0028]
Preferably, the aperture of the diaphragm is in the range of 1 μm to 10 μm.
[0029]
Preferably, the sample is scanned, a spectrum analyzed by the energy analyzer is stored in a memory for each position in the sample, and the spectrum stored in the memory is signal-processed to obtain a spectrum.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of an electron microscope according to the present invention will be described in detail with reference to the drawings. This embodiment assumes a scanning transmission electron microscope (STEM) equipped with an energy filter that simultaneously observes a high angle annular dark field (HAADF) using an annular detector. ing.
[0031]
FIG. 1 is a diagram showing a schematic configuration of an electron microscope according to the present embodiment.
[0032]
The electron microscope 10 includes an electron gun 11, an anode 12, a focusing lens 13, a scanning coil 14, an objective and intermediate lens 15, an aperture 16, and a direction in which an electron beam travels along the optical axis L0. An energy analyzer 17, a slit 18, a projection lens 19, and a recording observation device 20 are included. The objective and intermediate lens 15 includes an objective lens 15a and an intermediate lens 15b. Note that the locus L of the electron beam is shown in the figure.
[0033]
The electron microscope 10 has an annular detector (not shown) at a predetermined position (described later) along the optical axis L0 of the electron beam. In the electron microscope 10, a high-angle dark field image (HAADF) can be observed using an annular detector at the same time as observing a microscopic image of an electron beam having a specific energy selected by the energy analyzer 17.
[0034]
In the present embodiment, a field emission gun (FEG) is used as the electron gun 11. The energy analyzer 17 uses an omega filter.
[0035]
FIG. 2 is a diagram showing an energy analyzer using an omega filter.
[0036]
The omega filter has first to fourth magnetic poles 17a, 17b, 17c, and 17d. The first to fourth magnetic poles 17a to 17d generate a magnetic field by a wound coil, and the electron beam is symmetric. Deflection is performed so as to draw an Ω-shaped locus L symmetrically with respect to A. There are a plurality of electron beam trajectories L because the trajectory L drawn depends on the energy of the electron beam.
[0037]
In the figure, an incident window surface 111 where an electron beam incident on the omega filter crosses over, an incident image surface 112 which is an object surface conjugate to the object surface of the recording and observation apparatus 20, and electron beams having different energies. The positions of the exit image plane 113 that is imaged at the same position and also referred to as an achromatic image plane, and also referred to as an energy dispersion plane, and the exit window plane 114 on which the slit 18 is disposed are shown.
[0038]
The positions of the entrance window surface 111, the entrance image surface 112, the exit image surface 113, and the exit window surface 114 are important as basic optical positions of the electron microscope 10 including the energy analyzer 17. These positions are strictly determined in the design of the energy analyzer 17 in order to minimize the imaging distortion of the energy analyzer 17 and to ensure the same microscopic image observation function as a normal electron microscope without the energy analyzer 17.
[0039]
In the present embodiment, an omega filter is used for the energy analyzer 17, but other energy filters such as an alpha filter, a gamma filter, a mandolin filter, and a Wien filter can also be used. Note that the positions of the entrance window surface, the entrance image surface, the exit image surface, and the exit window surface are defined in any energy filter, not limited to the omega filter.
[0040]
In the present embodiment, a CCD camera is used as the recording observation device 20, but a recording film, an imaging plate, or the like can also be used.
[0041]
The electron beam emitted from the electron gun 11 is accelerated by a potential difference between the electron gun 11 and the anode 12 and is converged as a thin electron beam on the sample 101 by the focusing lens 13. At that time, the scanning coil 14 scans the position converged on the sample 101. The electron beam scanned on the sample 101 is converged by the objective and the intermediate lens 15 and converged on the incident window surface of the energy analyzer 17 through the opening 16. Of the electron beams that have passed through the energy analyzer 17, only an electron beam having a specific energy is selected by the slit 18 disposed on the exit window surface, and is projected onto the recording observation apparatus 20 by the projection lens 119.
[0042]
In the present embodiment, the electron microscope having the above-described configuration can ensure energy resolution by narrowing the electron beam on the entrance window surface of the energy analyzer 17 and obtain a dark field image by selecting diffracted light.
[0043]
FIG. 3 is a diagram illustrating the first embodiment.
[0044]
In the first embodiment, an energy resolution of 1 eV is assumed using an omega filter having an energy dispersion of 1.2 μm / eV at 200 kV as the energy analyzer 17.
[0045]
FIG. 4 is a diagram showing the shape of the electron beam on the exit window surface and the exit image plane as a reference example of the first embodiment.
[0046]
4A and 4B show the electrons on the exit window surface and the exit image surface when the diameter of the electron beam on the entrance window surface is 2.0 μm and the diameter of the electron beam on the entrance image surface is 2.5 μm. It is a figure which shows the shape of a beam, respectively. In addition, the spot in FIG.4 (b) is expanded 200 times.
[0047]
In the exit window shown in FIG. 4 (a), the right electron beam has no energy loss incident at an energy of 200 kV, and the left electron beam has lost energy by 1 eV, but they overlap. ing. Therefore, in order to obtain a resolution of 1 eV, it seems that the diameter of the electron beam should be 1 μm or less so that the right and left electron beams do not overlap. At this time, the diameter (1 μm) of the electron beam is 5/6 times the dispersion value (1.2 μm / eV).
[0048]
In the first embodiment, an annular detector 21 having an inner hole 21a having a diameter of 1 μm at the center is arranged in front of the entrance window surface 111 in the traveling direction of the electron beam along the optical axis L0. Along the optical axis L0, the distance D1 between the annular detector 21 and the incident window surface 111 is set to be half or more of the distance D2 between the incident window surface 111 and the incident image surface 112. According to such a configuration, the inner hole 21 a of the annular detector 21 functions as a diaphragm that determines the diameter of the electron beam on the incident window surface 111.
[0049]
Note that when an energy analyzer 17 other than the omega filter is used, the restriction that the width of the electron beam is 1 μm or less is also different. The energy analyzer has various dispersions, for example, 6 μm / eV for the mandolin filter and 10 μm / eV for the S-shaped filter, for example. Therefore, in order to obtain a resolution of 1 eV, it seems that the diameter of the electron beam is required to be 5 μm or less for the mandolin filter and 8.3 μm or less for the S-shaped filter. Here, considering the margin, the value of the electron beam diameter was obtained by multiplying the dispersion by 5/6 as in the case of the omega filter described above.
[0050]
Further, when the omega filter having a dispersion of 1.2 μm / eV of the present embodiment is used for the energy filter 17 and a resolution of 10 eV is required, it is considered that the diameter of the electron beam needs to be 10 μm or less. The diameter of the inner hole of the annular detector 21 serving as a diaphragm can be increased by a factor of 10 compared to the case where a resolution of 1 eV is required.
[0051]
FIG. 5 is a diagram illustrating the second embodiment.
[0052]
In the second embodiment, a microscope image of the sample 101 is formed on the incident window surface 111 and a diffraction image of the sample is formed on the incident image surface 112 by the electron beam incident from the intermediate lens 15b. Then, the annular detector 21 is disposed in the vicinity of the incident image plane 112 along the optical axis L0 of the electron beam. According to the present embodiment, a spectrum by EELS at a desired one point of the sample 101 can be acquired.
[0053]
The diffraction image formed on the incident image surface 112 does not move even when the electron beam is incident on the surface of the sample 101 by the scanning coil 16 because the electron beam is perpendicularly incident on the surface of the sample 101. The purpose of the annular detector 21 is to detect this diffraction image having the information of the sample 101 generated along with the scanning and obtain a high-angle dark field image (HAADF). The incident image surface 112 formed or its vicinity is desirable.
[0054]
FIG. 6 is a diagram showing the shape of the electron beam on the exit window surface and the exit image plane as a reference example of the second embodiment.
[0055]
6A and 6B show the electrons on the exit window surface and the exit image surface when the electron beam diameter on the entrance window surface is 0.1 μm and the electron beam diameter on the entrance image surface is 20.0 μm. It is a figure which shows the shape of a beam, respectively. The electron beam in FIG. 6 (a) is magnified 10 times, and the spot in FIG. 6 (b) is magnified 200 times.
[0056]
In the exit window shown in FIG. 6A, the right electron beam has no energy loss incident at an energy of 200 kV, and the left electron beam has lost energy by 1 eV.
[0057]
When a microscope image of the sample 101 is formed on the incident window surface 111, a diffraction image of the sample 101 is formed on the incident image surface 112. The diameter of the diffraction pattern of this sample can be made quite large.
[0058]
For example, the focal length fo = 2.3 mm of the objective lens 15a and the distance b = 192.3 mm between the main surface of the objective lens 15a and the image created by the objective lens 15a. Now, it is assumed that the radius of the electron beam irradiated onto the sample 101 is ro = 1 nm, and the convergence angle of the electron beam at that time is 30 mrad. However, the problem is an electron beam scattered from the sample 101 at a half angle of the emission angle ro ′ = 170 mrad.
[0059]
The magnification Mo of the objective lens 15a is 1− (b / fo) = 86.2, and the inclination angle ri ′ = ro / fo + ro ′ / Mo = 2.06 mrad of the electron beam after exiting the objective lens 15a. The size of the diffraction image formed on the focal plane of the lens 15a with respect to ro ′ = 170 mrad is rD = (b−fo) tan (ri ′) = 0.4 mm.
[0060]
If this diffracted image is projected onto the incident image plane 112 of the energy analyzer 17 with a camera length Lc = 20 mm, the magnification Md = Lc / fo = 8.7 between them, and thus the emission angle ro ′ = on the incident image plane 112. The radius of the electron beam corresponding to 170 mrad is rfi = rD × Md = 3.4 mm (diameter 6.8 mm).
[0061]
Assuming that the annular detector 21 is located between the incident window surface 111 and the incident image surface 112 of the energy analyzer 17, the diameter on the annular detector 21 is smaller than this. Thus, the annular detector 21 having a diameter of several millimeters is sufficiently realistic from the viewpoint of ease of manufacture and cost.
[0062]
Next, the diameter of the electron beam on the entrance window surface 111 is obtained. When the magnification of the intermediate lens 15b in the latter stage of the objective lens 15a and in the former stage of the energy analyzer 17 is Mi, and the distance between the incident window surface 111 and the incident image surface 112 is L1 = 95 mm, the magnitude of the magnification Mi is given by Sought by. Md × Mi = (b−fo) /L1=0.52. In this case, Mi = 0.52 / 8.7 = 0.06. Therefore, the radius of the electron beam on the entrance window surface 111 is rfd = ro × Mo × Mi = 1 × 82.6 × 0.05 = 5 nm (diameter 0.01 μm). This is a sufficiently small value, electron beams whose energies differ by 1 eV are sufficiently separated, and the energy resolution is higher than 0.5 eV. The shape of the electron beam has almost no bulge in the direction perpendicular to the dispersion and cannot be seen well in the figure. Therefore, in FIG. 6A, the diameter of the electron beam is shown as 0.1 μm and 10 times. Thus, when measuring the spectrum by EELS with the electron beam fixed at one point, an image having sufficient energy resolution can be obtained by projecting the microscope image of the sample 101 onto the entrance window surface 111.
[0063]
FIG. 7 is a diagram for explaining the third embodiment. FIG. 7A is a diagram illustrating pixels in the recording observation apparatus 20, FIG. 7B is a diagram illustrating spectra detected in each pixel, and FIG. 7C is a spectrum in which the positions of the pixels are adjusted. FIG. In the recording observation apparatus, pixels serving as detection units are two-dimensionally arranged.
[0064]
The third embodiment has the same configuration as that of the second embodiment, but the method for processing the diffraction image of the sample 101 detected by the recording observation apparatus 20 is different. That is, as in the second embodiment, a microscope image and a diffraction image of the sample 101 are formed on the incident window surface 111 and the incident image surface 112, respectively, but the diffraction image detected by the recording observation apparatus 20 is signal-processed.
[0065]
In the third embodiment, the EELS spectrum at each position of the electron beam scanned on the sample 101 by the scanning coil 16 is stored in the memory without being integrated. In the recording observation apparatus 20 shown in FIG. 7A, the spectrum detected for each pixel p1, p2, p3, p4... Is stored in a memory.
[0066]
In the present embodiment, the energy analyzer 17 is used to change the energy of the electron beam for each pixel p1, p2,... And measure the intensity of the electron beam, so that each pixel p1, p2 as shown in FIG. The spectrum by EELS is detected. In accordance with the movement of the position of the electron beam that scans the sample 101, the electron beam on the annular detector 21 also moves, and the position of the viewpoint of the spectrum also moves accordingly. For this reason, in the drawing, the position of the viewpoint on the horizontal axis is shifted according to the position.
[0067]
Detected spectra s1, s2,... Have high resolution at each position where the sample 101 is scanned. In the figure, the horizontal axis of the spectra s1, s2,... Is, for example, energy or wavelength, and the vertical axis is, for example, intensity.
[0068]
After the spectra s1, s2,... At the respective pixels p1, p2,... Are stored in the memory, this position is corrected as shown in FIG. That is, the position is corrected so as to return the shift of the spectrum position due to the scanning of the electron beam in the sample 101. If the spectra are then superimposed, the spectra can be integrated while maintaining high resolution.
[0069]
In addition, the above-mentioned embodiment shows the specific example of this invention, and this invention is not limited to this. The present invention can be implemented in various modes without departing from the scope of the present invention.
[0070]
【The invention's effect】
As described above, an object of the present invention is to provide a scanning transmission electron microscope including an energy analyzer, which includes an annular detector.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an electron microscope according to the present embodiment.
FIG. 2 is a diagram showing an energy analyzer using an omega filter.
FIG. 3 is a diagram illustrating a first embodiment.
FIG. 4 is a diagram illustrating a reference example of the first embodiment.
FIG. 5 is a diagram showing a second embodiment.
FIG. 6 is a diagram illustrating a reference example of the second embodiment.
FIG. 7 is a diagram for explaining a third embodiment;
FIG. 8 is a diagram illustrating a configuration of an energy filter including an annular detector.
FIG. 9 is a diagram for explaining an emission angle of an electron beam and a diameter of the electron beam at an incident window surface.
FIG. 10 is a diagram showing the shape of an electron beam on the exit window surface and the exit image surface of the energy analyzer.
FIG. 11 is a diagram for explaining the diameter of an electron beam on an incident window surface.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Electron microscope 11 Electron gun 12 Anode 13 Focusing lens 14 Scanning coil 15 Objective and intermediate lens 16 Aperture 17 Energy analyzer 18 Slit 19 Projection lens 20 Recording observation apparatus

Claims (3)

In an electron microscope that scans a sample with an electron beam and observes a transmission image,
An energy analyzer that analyzes the electron beam that has scanned the sample for energy;
An annular detector that is arranged in front of an incident window surface that crosses over the electron beam incident on the energy analyzer with respect to the traveling direction of the electron beam, and detects an electron beam scattered at a large angle by the sample;
Have
An electron microscope characterized in that a diffraction image of the sample is projected onto an incident window surface of the energy analyzer, and a spectrum or a microscope image is created using an electron beam analyzed by the energy analyzer.   The electron microscope according to claim 1, wherein a distance between the annular detector and the incident window surface is at least half of a distance between the incident window surface and the incident image surface. In an electron microscope that scans a sample with an electron beam and observes a transmission image,
An energy analyzer that analyzes the electron beam that has scanned the sample for energy;
An annular detector for detecting an electron beam scattered at a large angle by the sample, the electron beam being incident on the energy analyzer is disposed between an incident window surface connecting the crossover and an incident image surface with respect to a traveling direction of the electron beam. ,
Have
A microscopic image of the sample is formed on the incident window surface of the energy analyzer, a diffraction image of the sample is formed on the incident image surface, and a spectrum or a microscopic image is created using the electron beam analyzed by the energy analyzer. An electron microscope.

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