JP2015023010A - Electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmission electron microscope capable of accurately and easily obtaining a diffraction pattern in a nano region without electron beam damage.SOLUTION: A transmission electron microscope includes: an object lens that irradiates a sample with an electron beam emitted from an electron source and converges transmission electrons transmitted through the sample; a detector that detects the transmission electrons transmitted through the sample; a limit aperture that is provided below the object lens and limits the transmission electrons; an electron beam deflector that is provided above the sample and deflects the electron beam; an image shift deflector that is provided below the object lens and deflects the transmission electrons; and a control device that deflects the electron beam using the electron beam deflector so that an arbitrary measurement region of the sample is irradiated with the electron beam, and adjusts a deflection amount of the image shift deflector so that the electron beam transmitted through the sample passes through a hole of the limit aperture.

Description

  The present invention relates to an electron microscope, and more particularly to a transmission electron microscope capable of forming and observing and recording a transmission electron beam image and an electron diffraction pattern.

  One of the means for analyzing the crystal structure of a sample using a transmission electron microscope is a method using an electron beam diffraction pattern. There are two main methods for obtaining an electron diffraction pattern. One is a limited-field electron diffraction method, in which the target region is limited by a diaphragm, and the electron diffraction pattern from that region is magnified and observed with an imaging lens system, and the other is a nanoprobe electron. This is a line diffraction method, which is a nanoprobe electron beam diffraction method in which an electron beam is converged to the size of a target region of a sample by an irradiation lens, and an electron beam diffraction pattern in that region is enlarged and observed by an imaging lens system.

  In general, in the crystal structure analysis of nanomaterials, since the target region is on the order of nanometers, nanoprobe electron diffraction is used. In the nanoprobe electron beam diffraction method, the field of analysis is confirmed with the electron beam expanded, and the electron beam is converged toward the target to obtain an electron diffraction pattern. In order to converge the electron beam, the electron dose applied to the target region is large. Moreover, since the converged electron beam is used, the diffraction spots of the obtained diffraction pattern are not spot-like but disc-like. For this reason, the analysis accuracy is low as compared with the limited-field electron diffraction method in which diffraction spots are obtained in a spot shape.

  On the other hand, in the limited field diffraction method, the beam is expanded and a desired field of view is selected with a diaphragm, so that the irradiation electron beam density can be reduced and a high-resolution diffraction pattern can be obtained. However, it has been difficult to produce a fine aperture capable of limiting the nanometer order, and the nanoprobe electron diffraction method has generally been used for crystal structure analysis of nanomaterials.

  In recent years, with the progress of processing technology, it has become possible to produce a fine aperture that can limit a nanometer-order region using a focused ion beam (FIB), electrolytic polishing, or the like. Patent Document 1 describes a method of manufacturing a fine aperture using FIB. Patent Document 2 also describes a method for manufacturing a fine aperture. By using this micro-aperture, it has become possible to capture an electron beam diffraction pattern in a limited nano region without changing the electron beam irradiation conditions.

  In the limited field diffraction method, the size of the region on the sample surface that can be selected basically depends on the size of the limited field stop used and the magnification of the transmitted image projected on the stop.

JP 2011-243540 A JP 2006-331901 A

  In the above prior art, when the catalyst material analysis is performed using the nanoprobe electron diffraction method, the relationship between the crystal orientation of the catalyst particle having a particle diameter of 5 nm to 10 nm and the crystal orientation of the support on which the catalyst particle is supported is examined. Make a relationship. In this case, there is a problem that when the electron beam converged on the catalyst particles is irradiated, the movement direction of the catalyst particles is changed because a high-density electron beam is irradiated.

  Further, since the carrier particles are generally easily damaged by electron beam irradiation such as oxide, there is a problem that the carrier particles are damaged by the converged electron beam when acquiring the diffraction pattern.

  Furthermore, for example, when evaluating a sample in which various particles are mixed, statistical evaluation is necessary, and it is necessary to record as many electron diffraction patterns as possible corresponding to the transmission electron images of the particles. It is necessary to acquire electron diffraction patterns for a large number of individual catalyst particles. However, since it is necessary to confirm the position, it is necessary to change the irradiation lens current to widen the electron beam, and after confirming the particle position, it is necessary to narrow down the electron beam with respect to the particle. there were.

  On the other hand, in the limited field diffraction method, even if the minute limited field stop manufactured by FIB described in Patent Document 1 is used, it is difficult to avoid sample drift due to sample movement because the object is nanometer size. there were. That is, even if the sample stage is moved so that transmitted electrons in the sample region to be observed pass through the limited field stop, there is a problem that the sample stage does not stop at the target position due to drift. The smaller the hole in the limited field stop, the more likely this problem occurs.

  An object of the present invention is to provide a transmission electron microscope capable of solving the above-described problems and accurately and easily acquiring a diffraction pattern in a nano region without damaging an electron beam.

  In order to solve the above problems, the present invention has the following configuration. An objective lens that includes an objective lens that irradiates a sample with an electron beam emitted from an electron source and focuses the transmitted electrons that have passed through the sample, and a detector that detects the transmitted electrons that have passed through the sample. A limiting aperture for limiting the transmitted electrons below, an electron beam deflector for deflecting an electron beam above the sample, and an image shift deflector for deflecting the transmitted electrons below the objective lens, The electron beam deflector deflects the electron beam so that an arbitrary measurement region is irradiated with the electron beam, and the image shift deflection causes the electron beam transmitted through the sample to pass through the hole of the limiting aperture. A transmission electron microscope comprising a control device for adjusting the deflection amount of the instrument.

  According to the present invention, an electron beam diffraction pattern of a target sample region can be obtained using a limited-field electron beam diffraction method in a state where sample damage is reduced by a low electron beam dose.

1 is a basic configuration diagram of a transmission electron microscope 1 according to an embodiment of the present invention. Optical path diagrams of transmission electron image observation mode (a) and limited-field electron beam diffraction mode (b) in a transmission electron microscope as an explanatory diagram Optical path diagrams of limited-field electron diffraction mode (a) and nanoprobe electron diffraction mode (b) in the transmission electron microscope, which is an explanatory diagram FIG. 1 is a configuration diagram in the vicinity of an objective lens according to an embodiment of the present invention. A micro-region limited field-of-view image (a) and an electron diffraction pattern (b) according to an embodiment Display example of limited field of view when acquiring a plurality of limited field diffraction patterns according to one embodiment Procedure for obtaining a plurality of minute limited field diffraction patterns which is one of the embodiments

  FIG. 1 shows a basic configuration diagram of a transmission electron microscope 1 according to an embodiment of the present invention. The mirror body of the transmission electron microscope 1 includes an electron gun 2, a condenser lens 3, an objective lens 4, intermediate lenses 5a and 5b, and projection lenses 6a and 6b. Each lens is connected to a lens power source controller 8 via a lens excitation power source 7.

  A sample holder 9 for an electron microscope is inserted between the objective lenses 4. A sample 10 is mounted on the sample holder 9, and the position control of the sample 10 is controlled by a sample fine movement control unit 11. A fluorescent screen 12 is mounted below the projection lens 6b, and a TV camera 13 is mounted below the fluorescent screen 12. The TV camera 13 is connected to the image display unit 15 and the image storage device 16 via the image recording control unit 14.

  The electron beam 17 generated from the electron gun 2 is converged by the condenser lens 3 and irradiated onto the sample 10. The electron beam 2 transmitted through the sample 10 is imaged by the objective lens 4, magnified by the intermediate lenses 5 a and 5 b and the projection lenses 6 a and b, and projected onto the fluorescent screen 12. When the fluorescent screen 12 is lifted, it is projected onto the TV camera 13, and a transmission image is displayed on the image display unit 15 and recorded in the image storage device 16.

  The image plane of the objective lens is provided with a limited field stop 18 having a stop hole with a hole diameter of 1 micron or less. The limited field stop 18 can be moved in the horizontal direction, and can be taken in and out on the optical axis in accordance with the observation target. The position control of the limited field stop 18 is motor driven and is controlled by the limited field stop control unit 19. One or more stages of beam shift deflection coils 20 are provided above the sample 10, and one or more stages of image shift deflection coils 21 are also provided below the objective lens 4 and above the limited field stop 18. Both deflection coils are connected to deflection coil controllers 22a and 22b. The lens power supply control unit 8, the sample fine movement control unit 11, the image recording control unit 14, the limited field stop control unit 19, and the deflection coil control units 22a and 22b are connected to the microprocessor 23. The microprocessor 23 controls the entire apparatus. .

  FIGS. 2A and 2B are optical path diagrams of a transmission electron microscope during transmission electron image observation and electron beam diffraction pattern observation, which are explanatory diagrams of the present invention.

  During transmission electron image observation, the electron beam 17 generated from the electron gun 2 is irradiated onto the sample 10 in parallel by the condenser lens 3. When the sample 10 is a crystalline sample, there are an electron beam that goes straight without being diffracted by the crystal and an electron beam that is diffracted, and the electron beam diffracted at the same angle is at the same point on the back focal plane of the objective lens 4. Collectively, an electron diffraction pattern 24a is formed on the back focal plane. The electron beam 17 on which these electron beam diffraction patterns 24 a are formed further forms a transmission electron image 25 a on the image plane of the objective lens 4 and the position of the limited field stop 18. This image is further magnified by the intermediate lens 5 and the projection lens 6, and a transmission electron beam image 25b is projected onto the fluorescent screen 12 or the TV camera 13.

  On the other hand, when observing the electron beam diffraction pattern 24a, the intermediate lens 5 is focused on the electron beam diffraction pattern 24a formed at the back focal point of the objective lens 4 and magnified by the intermediate lens 5 and the projection lens 6, and the electron beam The diffraction pattern 24 b is projected onto the fluorescent screen 13 or the TV camera 13.

  FIGS. 3A and 3B show optical path diagrams of a limited field diffraction method and a nanoprobe diffraction method, which are explanatory diagrams of the present invention. In the limited field diffraction method, the electron beam 17 parallel to the sample 10 is irradiated, and a region where a diffraction image is obtained by the limited field stop 18 is determined. On the other hand, since the focused electron beam 17 is used in the nanoprobe diffraction method, the amount of electrons irradiated onto the sample 10 is large. The nanoprobe diffraction method has a problem that the spot of the obtained electron beam diffraction pattern 24b has a disk shape, which makes analysis with high accuracy difficult.

  FIG. 4 shows a configuration diagram in the vicinity of the objective lens 4 according to an embodiment of the present invention. A beam shift deflection coil 20 (x direction is 20a and y direction is 20b) is connected from an adder 27 to a digital-analog converter (hereinafter referred to as DAC) DAC 28a through an amplifier 26a. The image shift deflection coil 21 (21a in the x direction and 21b in the y direction) is connected to the DAC 28b via the amplifier 26b. The output of the DAC 28b is also connected to the adder 27. Both the DAC 28a and the DAC 28b are connected to the BUS 29 and controlled by a computer. With such a configuration, when certain data is written to the DAC 28b for visual field shift (movement), a current is output to the image shift deflection coil 21, and at the same time, a certain amount proportional to the written data is present. The signal is also output to the beam shift deflection coil via the adder 27. With this proportional amount, the weight of the adder is set so that the shift amount of the image (field of view) and the shift amount of the beam irradiating the sample are the same. That is, the image shift and the beam shift are linked to form the same field of view.

  When observing an electron beam diffraction pattern by a normally limited field electron diffraction method, when the electron beam 17 is irradiated in parallel to the sample 10, a limited portion of an image 25a formed on the surface of the limited field stop 18 by the objective lens 4 Only the information of the region (dotted arrow part 30a) on the sample surface corresponding to (dotted arrow part 30b) is selected, and the electron beam diffraction pattern 24a formed by those selected electron beams is observed. When obtaining a diffraction pattern of another visual field region (white arrow 31a), the visual field (image) is moved using the image shift deflection coil 21. Since the beam is not irradiated to the field of view as it is, it is necessary to move the beam using the beam shift deflection coil 20, but both shift amounts are interlocked by the adder 27 as described above. , Always the same field of view. That is, a transmission enlarged image (white arrow 31b) by the objective lens 4 is formed on the limited field stop 18, but is shifted from the position of the limited field stop 18 hole. When the image shift deflection coils 21a and 21b are operated so as to deviate so as to enter the limited field stop 18, the position of the field area irradiated with the beam shift deflection coil 20 is corrected (white arrow 31a). Then, the corrected transmission enlarged image (white arrow 31c) is moved to the position of the limited field stop 18 hole. As a result, an electron beam diffraction pattern 24a corresponding to the visual field area of the white arrow 31a is imaged on the back focal plane of the objective lens 4.

  According to this method, since the limited field stop 18 and the sample 10 are not mechanically moved, it is possible to obtain the electron diffraction pattern 24b in a minute region without drift due to mechanical vibration. When the position is largely changed, the position larger than the beam deflectable region can be obtained by combining the movement of the sample 10 and the limited field stop 18 and the field movement by the beam shift deflection coil 20 and the image shift deflection coil 21. It is possible to move.

FIG. 5 shows a limited field image (a) of a minute region and an electron diffraction pattern (b) thereof as an example. A transmission image 25b of tin oxide (SnO ₂ ) in a region of about 12 nm in a state where a limited field stop 18 having a minute hole having a 1 micron diameter is inserted, and an electron beam diffraction pattern 24b thereof are shown. Even when the limited field stop 18 is not inserted, if a target area is selected during observation, the size of the limited field movable diaphragm 18 is displayed on the image, and the limited field stop 18 is actually inserted in the displayed area. Then, an electron beam diffraction pattern 24b is obtained. At this time, the positional information of the restricted area and the electron beam diffraction pattern 24b are stored in pairs. From the distance (R) between the diffraction spots of the obtained electron beam diffraction pattern 24b, the crystal lattice spacing (d) of the target sample 10 is obtained from dR = Lλ. Here, L is the camera length, λ is the wavelength of the electron beam, and Lλ is a constant. The determined values are listed and sorted together with the limited field image and the electron diffraction pattern.

  In the conventional technology, it is possible to record the area limited by the limited field stop together with the image of the stop, but display and record the stop position on the whole image, and simultaneously acquire the diffraction pattern corresponding to the stop position. The points to be recorded were not considered. Further, the grid intervals obtained from the obtained diffraction patterns are sorted, and the region having the same grid interval in each limited field of view is separated and displayed. In the above prior art, the influence of the spherical aberration of the objective lens of the limited field image is not taken into consideration.

  The present embodiment is to provide a transmission electron microscope capable of accurately displaying and recording a limited visual field on a TEM image and separately displaying and recording regions having the same crystal structure interval.

  FIG. 6 shows a display example of a limited visual field when acquiring a plurality of electron beam diffraction patterns 24b using the present invention. FIG. 6A is a schematic diagram of a transmission image on the image display unit 15. Here, black circles and white triangles in the figure are transmission electron images of particles having different crystal plane intervals.

  First, a desired region in which the electron beam diffraction pattern 24b is desired is moved to the center of the screen display unit 15 by the sample fine movement control unit, and an image is recorded. The center of the screen display unit 15 is set as the origin, and the origin is selected with the cursor 32. A circle 33 reflecting the diameter of the limited field stop 18 is displayed and labeled on the screen display unit 15 (here, label 1 is set). In FIG. 6B, another region for which a diffraction pattern is desired to be obtained is selected with a cursor and labeled (here, label 2 is set). In FIG. 6C, up to label 12 is set. The selected position information is transferred to the label and the microprocessor. In the microprocessor, the beam shift deflection coil 20 and the image shift deflection coil 21 are arranged so that the transmission image 31b on the restriction field stop 18 plate moves to the transmission image 31c at the position of the restriction field stop 18 so as to correspond to the position information. Make it work. The obtained electron diffraction pattern 24b is stored in the image storage device together with the label. For the electron diffraction pattern 24b, the lattice plane spacing is obtained, and the lattice spacing for the label is obtained. Create a list of grid spacings for the labels (d). The same ones (for example, crystal structure analysis of substances A and B) are classified from the obtained lattice spacing, and the limited visual field is classified by color (classified by dotted lines and solid lines in the embodiment) on the display screen (d). Here, labeling is performed first, and electron diffraction patterns are acquired in that order, but an electron diffraction pattern may be acquired for each labeling and limited field display, and a lattice spacing list may be created.

The limited visual field to be displayed may be a region obtained by adding an error Y in the visual field selection to the aperture diameter so that whether or not the error Y can be captured can be selected. When the spherical aberration coefficient of the objective lens is Cs, the Bragg angle of the crystal plane to be observed is θ _B , and the error in focusing is D, the error Y in the field selection is

Given in. That is, an error of Y is expected in the limited visual field.

  FIG. 7 shows a procedure for obtaining an electron beam diffraction pattern 24b of a plurality of measurement objects, which is one of the present embodiments. After setting the intermediate lens 5 and the projection lens 6 so that the transmission image is projected onto the TV camera 13, the following steps are executed.

  (1) First, the transmission image 25b of the sample 10 is acquired by the TV camera 13, and displayed and recorded on the image display unit 15. At this time, a transmission image is recorded by setting the region where the first electron beam diffraction pattern 24b is desired to be obtained at the center (origin) of the selected visual field by the sample fine movement control unit to be the center of the screen.

  (2) Select the field of view to obtain the electron diffraction pattern 24b with the cursor 32.

  (3) The position information of the cursor 32 is labeled for each selected field of view and stored in the microprocessor 23, and the limited field of view corresponding to the magnification is displayed on the display unit as a circle 33 with the label at the center of the selected cursor 32 position. To do.

  (4) The field of view in the transmission image is selected.

  (5) Insert the limited field stop 18 in the center of the screen.

  (6) The intermediate lens 5 and the projection lens 6 are set so that the electron beam diffraction pattern 24b of the label 1 is projected onto the TV camera 13, and the electron beam diffraction pattern 24b of the label 1 is displayed and recorded. For the other field of view, the microprocessor 23 instructs the deflection coil controller 22 to move the beam shift deflection coil 20 and the image shift deflection coil 21 based on the position information with respect to the label 1. Similarly, the individual limited electron diffraction patterns labeled are recorded and displayed.

  (7) Next, in the microprocessor 23, the diffraction spot interval is measured for each electron beam diffraction pattern, and the lattice plane interval is calculated and recorded.

  (8) In the microprocessor, the lattice plane spacing calculated for each label is listed and displayed on the image display unit 15.

  (9) The listed lattice spacings are classified by the microprocessor 23.

  (10) The restricted visual field is displayed in different colors on the image display unit 15 for each classified lattice plane interval. As a result, it is possible to acquire and record a limited field electron beam diffraction pattern of a minute region with suppressed influence of the electron beam, and to acquire an accurate distribution of lattice plane spacing.

  In FIG. 4, the electron beam 17 is deflected by using the electron beam deflection coil 20 and the image shift deflection coil 21 to obtain a plurality of limited visual fields, but electron beam diffraction in a region beyond the deflectable region. In order to obtain a pattern, the sample fine movement is used together, the sample 10 is moved by the deflectable region, and then a beam shift deflection coil 20 and an image shift deflection coil 21 are used to obtain an electron diffraction pattern of a new region. You may make it acquire. Further, instead of using the sample fine movement together, the limited field stop 18 may be moved by a motor drive. In this case, since the limited visual field position deviates from the optical axis, the electron beam 17 incident on the intermediate lens 5 also deviates from the optical axis. Therefore, the beam shift deflection coil 20 and the image shift are located below the intermediate lens 5 upper limited visual field stop 18. A deflection coil 21 may be provided so that the transmission electron beam 17 incident on the intermediate lens 5 is incident on the intermediate lens 5 at the center and perpendicularly.

  According to each of the above embodiments, the diffraction pattern corresponding to the limited visual field can be stored in pairs, and the limited visual field can be visually recognized. Furthermore, for the same crystal lattice spacing obtained from the diffraction pattern, the corresponding limited field of view can be separated and identified by the color difference on the TEM image, improving the efficiency of crystal structure analysis for a large number of minute regions. .

DESCRIPTION OF SYMBOLS 1 Transmission electron microscope 2 Electron gun 3 Condenser lens 4 Objective lens 5 Intermediate lens 6 Projection lens 7 Lens excitation power supply 8 Lens power supply control part 9 Sample holder 10 Sample
11 Sample fine movement control unit 12 Fluorescent screen 13 TV camera 14 Image recording control unit 15 Image display unit 16 Image storage device 17 Electron beam 18 Restricted field stop 19 Restricted field stop control unit 20 Beam shift deflection coil 21 Image shift deflection coil 22a, 22b Deflection coil controller 23 Microprocessor 24a Electron diffraction pattern 24b formed on the back focal plane of the objective lens 4 Electron diffraction pattern 25 projected on the fluorescent screen 12 and the TV camera 13 Transmission electron images 26a and 26b Amplifier 27 Adder 28a , 28b Digital-to-analog converter (DAC)
29 BUS
30a Area on sample surface (dotted line arrow)
30b Restricted portion of the image 25b formed on the surface of the restricted field stop 18 (dotted line arrow portion)
31a Another area on the sample surface (white arrow)
31b Transmission image on white plate with limited field stop 18 (white arrow)
31c Transmission image (dotted line arrow) corrected to the position of the limited field stop 18 holes
32 Cursor 33 A circle reflecting the 18-diameter limited field stop centered on the cursor

Claims (7)

In a transmission electron microscope provided with an objective lens that irradiates a sample with an electron beam emitted from an electron source and focuses the transmitted electrons that have passed through the sample, and a detector that detects the transmitted electrons that have passed through the sample.
A limiting aperture for limiting the transmitted electrons under the objective lens;
An electron beam deflector that deflects an electron beam on top of the sample;
An image shift deflector for deflecting transmitted electrons at the bottom of the objective lens;
The electron beam deflector deflects the electron beam so that an arbitrary measurement region of the sample is irradiated with the electron beam, and the electron beam transmitted through the sample passes through the hole of the restriction aperture. A transmission electron microscope comprising a control device for adjusting a deflection amount of an image shift deflector. The transmission electron microscope of claim 1,
A transmission electron microscope characterized in that a diffraction image of an electron beam irradiation region of the sample is acquired and the sample is analyzed from the diffraction image. The transmission electron microscope of claim 1,
Comprising designation means for designating a region to be measured of the transmission electron image of the sample;
A transmission electron microscope characterized by displaying the designation means on a display screen for displaying the transmission electron image. The transmission electron microscope of claim 3,
The transmission electron microscope characterized in that the electron beam deflector deflects an electron beam so as to irradiate the electron beam onto a sample portion designated by the designation means. The transmission electron microscope of claim 3,
The designation means can designate a plurality of measurement locations on the sample,
The transmission electron microscope characterized in that the electron beam deflector deflects an electron beam so as to irradiate the electron beam onto a sample portion designated by the designation means. The transmission electron microscope of claim 5,
Analyzing the substance based on the electron diffraction pattern of the plurality of measurement locations,
A transmission electron microscope characterized in that the plurality of measurement locations are classified and displayed on the display screen. The transmission electron microscope of claim 1,
A transmission electron microscope comprising a driving mechanism for driving the sample or the limiting diaphragm.