JP4891736B2 - Ronchigram center determination method - Google Patents

Description

  The present invention relates to a method for determining the center of a Ronchigram, and in particular, in a scanning transmission electron microscope (STEM), a plurality of Ronchigram images are photographed by changing an acceleration voltage or an objective lens current to correct aberrations and adjust axes. On how to determine.

  In a scanning transmission electron microscope, a Ronchigram is often used for axis adjustment and aberration correction. Here, the Ronchigram is an image of an electron beam diffraction pattern formed along the optical axis in the electron diffraction pattern transmitted through the sample in the STEM. However, which position (axis) on the Ronchigram is centered on which aberration correction is performed is arbitrary, and in existing automatic aberration correction apparatuses, the method for determining the axis center is manual, and is highly optional. There is also a method for obtaining the center of the axis while looking at the beam, but the influence of the imaging system comes out.

  As a conventional device of this type, for each coordinate of the image memory, the difference is obtained from the image data before the objective lens current or the acceleration voltage is changed and the image data after the change, and the coordinates of the image memory A technique is known in which an approximate function indicating a relationship with a difference is calculated, an image memory coordinate having a minimum difference is obtained by calculation based on the approximate function, and the current (or voltage) is the center of the coordinate. (For example, refer to Patent Document 1).

In addition, a technique is known in which image data is divided into a plurality of areas, and the movement amount of each divided area is obtained to detect the movement amount of the image data and obtain the center of the current axis or voltage axis. (For example, refer to Patent Document 2).
JP-A-4-192244 (page 3, lower right column, line 11 to page 4, line 10; FIGS. 1 and 2) JP 2001-210261 (paragraphs 0015 to 0024, FIGS. 2 and 3)

  In the prior art, it was difficult to determine the axis on the Ronchigram. Further, automatic aberration correction has not been taken into consideration including the way to set the voltage axis and current axis. The present invention has been made in view of such problems, and an object of the present invention is to provide a method for determining a Ronchigram center that can determine a Ronchigram center for aberration correction and axis adjustment.

(1) According to the first aspect of the present invention, an imaging means is provided on the final imaging surface of a scanning transmission electron microscope, a sample capable of recognizing the shape is arranged, and an optical system capable of observing a Ronchigram is sufficiently defocused. In this state, the Ronchigram image 1 is taken by the imaging means, the acceleration voltage is changed, the magnification change m is calculated from the defocus amount changed by the known chromatic aberration, and the Ronchigram image is obtained by the imaging means in this state. 2 is used, the magnifications of the Ronchigram image 1 and the Ronchigram image 2 are matched using the magnification change m, and the Ronchigram image 1 and the Ronchigram image 2 having the same magnification are cross-correlated to each other . It is characterized by obtaining a fixed point.
(2) According to the second aspect of the present invention, an imaging means is provided on the final imaging surface of the scanning transmission electron microscope, a sample capable of recognizing the shape is disposed, and an optical system capable of observing the Ronchigram is sufficiently defocused. In this state, the Ronchigram image 1 is taken by the imaging means, the objective lens current is changed in this state, and the magnification ratio change m is calculated from the changed defocus amount. In this state, the Ronchigram image is obtained. 2 is used, the magnifications of the Ronchigram image 1 and the Ronchigram image 2 are matched using the magnification change m, and the Ronchigram image 1 and the Ronchigram image 2 having the same magnification are cross-correlated to each other . It is characterized by obtaining a fixed point.
(3) The invention described in claim 3 provides sufficient defocusing by providing an imaging means on the final imaging surface of the scanning transmission electron microscope, placing a sample capable of recognizing the shape, and observing the Ronchigram. In this state, the Ronchigram image 1 is taken with the imaging means, the acceleration voltage is changed, and the magnification near the center of the aberration is made the same as when the Ronchigram image 1 is taken. The focus amount is compensated by the excitation of the objective lens or the sample height, and the same defocus as before the acceleration voltage change is made. In this state, the Ronchigram image 2 is taken by the imaging means, and the Ronchigram image 1 and the Ronchigram image 2 are used. A cross-correlation near the center of the aberration is taken, the deflector above the objective lens deflects the electron beam in the direction of the cross-correlation peak, and the Ronchigram image 1 Photographing the beam image 2, the cross-correlation peak location processing taking the cross correlation and repeating until the center.

(1) According to the first aspect of the present invention, the fixed point of the image can be obtained by performing a predetermined calculation process using the Ronchigram images before and after the acceleration voltage change.
(2) According to the invention described in claim 2, the fixed point of the image can be obtained by performing predetermined image processing using the Ronchigram images before and after the objective lens current change.
(3) According to the third aspect of the present invention, when the aberration center is known, the fixed point of the image can be obtained by performing predetermined image processing using the Ronchigram images before and after the acceleration voltage change. .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing a configuration example of an apparatus for carrying out the present invention. In the figure, reference numeral 10 denotes a scanning transmission electron microscope. In the scanning transmission electron microscope 10, 1 is an objective lens, 2 is a sample, 3 is a projection lens for receiving an electron beam transmitted through the sample 2 and forms an image on an image plane, and 4 is a final image plane. A camera 5 as an arranged imaging means is an arithmetic device that performs various calculations based on the image captured by the camera 4 and finds a fixed point. As the arithmetic unit 5, for example, a personal computer (PC) is used. In the figure, an electron beam source that emits an electron beam and a deflection system that deflects the electron beam are omitted. The present invention will be described using the thus configured apparatus.
(Example 1) Method for obtaining voltage center 1) The camera 4 is disposed on the final imaging plane of the scanning transmission electron microscope 10, and a sample 2 that can recognize the shape is used. This is because if the shape cannot be recognized, the correlation described later is difficult to take.
2) An optical system capable of observing the Ronchigram is taken and sufficient defocusing is taken. If the optical system is determined in this way, the chromatic aberration is constant and known.
3) In this state, the camera 4 is used to take a Ronchigram image 1 (Ronchi 1).
4) Next, the acceleration voltage is changed. When the acceleration voltage is changed, the magnified image moves radially around a certain position (voltage center).
5) The enlargement ratio change m is calculated from the defocus amount changed by the known chromatic aberration.

  FIG. 2 is an explanatory diagram for calculating the enlargement ratio change m. In the figure, 2 is a sample and 6 is a screen. The screen 6 is a surface on which the camera 4 of FIG. 1 is disposed. In FIG. 2, df is the defocus amount, and L is the distance from the sample 2 to the screen 6 (camera length). In general, L >> df. The enlargement ratio M near the center of the Ronchigram when defocus is large is expressed by the following equation.

M = L / df
Defocus df _Cc due to chromatic aberration when the acceleration voltage is changed is expressed by the following equation.
df _Cc = Cc × (ΔV / V)
Here, Cc is chromatic aberration, ΔV is an acceleration voltage change amount, and V is an acceleration voltage. Therefore, the enlargement ratio M ′ after the acceleration voltage change is expressed by the following equation.

M ′ = L / (df _Cc + df)
The enlargement ratio change m before and after the acceleration voltage change is expressed by the following equation.
m = M ′ / M = (df _Cc + df) / df = 1 + ((Cc × ΔV / V) ÷ df)
In actual measurement, m is preferably between 0.2 and 5 in view of image processing accuracy.
6) The Ronchigram image 2 (Ronchi 2) is photographed using the camera 4 in this state.
7) Using the arithmetic device 5 and using the launch 1 and the launch 2, taking into account the magnification change m, the fixed points of the two images are obtained.

  FIG. 3 is an explanatory diagram of how to obtain a fixed point. Ronchi 1 is a Ronchigram image 1 photographed before changing the acceleration voltage, and Ronchi 2 is a Ronchigram image 2 photographed after changing the acceleration voltage. As shown in the figure, a launch 1 is first obtained. Next, Ronchi 2 is obtained. Here, the launch 1 is multiplied by m to obtain an image A in which the launch 2 and the size of the image are combined. Then, a cross-correlation is taken between the image A and the launch 2. As a result, a cross correlation image B is obtained. C is a fixed point obtained in the cross-correlation image B. Here, a cross-correlation formula will be described. When the function of the image A is g (x) and the function of the launch 2 is f (x), the cross-correlation between these two images is expressed by the following equation.

Here, ◯ between g and f represents a cross-correlation operation, and “−” represents a complex conjugate (the same applies hereinafter). The point where the cross-correlation becomes the maximum value is the fixed point.
8) When the fixed point is obtained in step 7), since this fixed point is the center of the axis, aberration correction may be performed on this axis as necessary.

Thus, according to the first embodiment, the fixed point of the image can be obtained by performing the predetermined image processing using the Ronchigram images before and after the acceleration voltage change.
(Example 2) How to obtain the current center 1) The camera 4 is arranged on the final imaging plane of the scanning transmission electron microscope 10, and a sample 2 that can recognize the shape is used.
2) An optical system capable of observing the Ronchigram is taken and sufficient defocusing is taken. If the optical system is determined in this way, the chromatic aberration is constant and known.
3) In this state, the camera 4 is used to take a Ronchigram image 1 (Ronchi 1).
4) Next, the objective lens current is changed. When the objective lens current is changed, the magnified image moves in the circumferential direction around a certain position (current center). In this case, the changed defocus amount is known.
5) The enlargement ratio change m is calculated from the defocus amount changed by the known chromatic aberration. The calculation method of the enlargement ratio change m is the same as that described in 5) of the first embodiment.
6) In this state, the camera 4 is used to photograph the Ronchigram image 2 (Ronchi 2).
7) Using the arithmetic device 5, in the launch 1 and the launch 2, the fixed point of the two images is obtained in consideration of the magnification rate change m. The method of obtaining is the same as that described in the first embodiment.
8) When the fixed point is obtained in step 7), since this fixed point is the center of the axis, aberration correction may be performed on this axis as necessary.

According to the second embodiment, the fixed point of the image can be obtained by performing predetermined image processing using the Ronchigram images before and after the objective lens current change.
(Example 3) When the aberration center is known 1) The camera 4 is disposed on the final imaging surface of the scanning transmission electron microscope 10, and a sample 2 that can recognize the shape is used.
2) An optical system capable of observing the Ronchigram is taken and sufficient defocusing is taken. If the optical system is determined in this way, the chromatic aberration is constant and known.
3) In this state, the camera 4 is used to take a Ronchigram image 1 (Ronchi 1).
4) Next, the acceleration voltage is changed. When the acceleration voltage is changed, the magnified image moves radially around a certain position (voltage center).
5) In order to make the magnification near the center of the aberration (area where defocus is the main aberration) the same as 3), the defocus amount changed by the chromatic aberration is compensated by excitation of the objective lens 1 or the sample height. Use the same defocus as before the acceleration voltage change.
6) In this state, the camera 4 is used to photograph the Ronchigram image 2 (Ronchi 2).
7) Using the arithmetic unit 5, in the launches 1 and 2, the cross-correlation of the vicinity of the center of the aberration (area where defocus is the main aberration) is obtained. The cross correlation is the same as that described in the first embodiment.
8) An electron beam is inclined in the direction of the cross-correlation peak by a deflector (not shown) above the objective lens 1 and 3) to 7) are performed.
9) Repeat 3) to 8) until the cross-correlation peak position is at the center.
10) When the cross-correlation peak position is at the center, aberration correction is performed for the axis.

According to the third embodiment, the axis center when the aberration center is known can be obtained.
Next, another method for finding a fixed point will be described. FIG. 4 is an explanatory diagram of another method for obtaining a fixed point.
1) In the figure, E and F are Ronchigram images, and these two images are overlapped.
2) Find a feature point and draw a straight line connecting the feature points of the two images. Here, the feature point is a point that characterizes the image, and represents a feature such as a circle, a triangle, or a square. Since the launch E and the launch F are images of the same sample, the feature point of E and the feature point of F are common. In FIG. 4, K1 is a feature point of Ronchi E, and K2 is a feature point of Ronchi F. Here, a straight line L1 connecting K1 and K2 is drawn.
3) Find another feature point and draw a straight line in the same way. J1 is a feature point of Ronchi E, and J2 is a feature point of Ronchi F. Here, the point J1 and the point J2 are connected and a straight line L2 is drawn.
4) Find the intersection of the straight lines L1 and L2. In the figure, P is the intersection. This intersection P becomes a fixed point.

In this method, it is not always necessary to know the change m of the enlargement ratio, but it is necessary to find a feature point in the two images as a premise.
FIG. 5 is an explanatory diagram of how to obtain another fixed point by cross-correlation. 4 is obtained by adding the way of finding the fixed point shown in FIG. 4 to the way of finding the fixed point shown in FIG. An image obtained by multiplying Launch 1 by m is denoted as A ′. The image A ′ and the launch 2 are cross-correlated to obtain an image B ′. For example, a straight line connecting points coincided with the square H multiplied by m and the original square (Ronchi 1 square) G with the cross-correlation peak as the center is drawn. A point where two or more such straight lines intersect is a fixed point. In the example shown in FIG. 5, a point Q where a line 11 connecting the left ends of the squares G and H intersects with a line 12 connecting the centers is a fixed point.

  In the above-described embodiment, the case where the fixed point is obtained from the entire Ronchigram image has been described as an example. However, the present invention is not limited to this, and the analysis may be performed on a clipped region from the Ronchigram image. In the first and second embodiments described above, the method for finding the fixed point has been described. However, the present invention is not limited to this, and the work for finding the center may be repeated until convergence. Further, in the above-described embodiment, the magnification is assumed to be the change in magnification m due to the focus change. However, for example, the n-value width of the autocorrelation peak of the image may be compared to incorporate the magnification calculation into the algorithm. . Here, the autocorrelation is expressed by the following equation.

  As described above, according to the present invention, it is possible to provide a method of determining a Ronchigram center that can determine a Ronchigram center for aberration correction and axis adjustment.

It is a figure which shows the apparatus structural example which implements this invention. It is explanatory drawing of magnification ratio change m calculation. It is explanatory drawing of how to obtain a fixed point. It is explanatory drawing of how to obtain | require another fixed point. It is explanatory drawing of how to obtain | require another fixed point by cross correlation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Sample 3 Projection lens 4 Camera 5 Arithmetic apparatus 10 Scanning transmission electron microscope

Claims (3)

An imaging means is provided on the final imaging surface of the scanning transmission electron microscope, a sample whose shape can be recognized is placed, and an optical system that can observe the Ronchigram is defocused sufficiently.
In this state, the Ronchigram image 1 is taken with the imaging means,
Change the acceleration voltage,
Calculate the magnification change m from the defocus amount changed by known chromatic aberration,
In this state, the Ronchigram image 2 is taken by the imaging means,
The magnification of the Ronchigram image 1 and the Ronchigram image 2 is matched using the magnification change m, and the fixed point of the image is obtained by taking the cross-correlation between the Ronchigram image 1 and the Ronchigram image 2 having the same magnification. A method for determining the center of a Ronchigram. An imaging means is provided on the final imaging surface of the scanning transmission electron microscope, a sample whose shape can be recognized is placed,
Use an optical system that can observe Ronchigrams, take enough defocus,
In this state, the Ronchigram image 1 is taken with the imaging means,
In this state, change the objective lens current,
Calculate the magnification change m from the changed defocus amount,
In this state, the Ronchigram image 2 is taken by the imaging means,
The magnification of the Ronchigram image 1 and the Ronchigram image 2 is matched using the magnification change m, and the fixed point of the image is obtained by taking the cross-correlation between the Ronchigram image 1 and the Ronchigram image 2 having the same magnification. A method for determining the center of a Ronchigram. An imaging means is provided on the final imaging surface of the scanning transmission electron microscope, a sample whose shape can be recognized is placed, and an optical system that can observe the Ronchigram is defocused sufficiently.
In this state, the Ronchigram image 1 is taken with the imaging means,
Change the acceleration voltage,
In order to make the magnification near the center of the aberration the same as when the Ronchigram image 1 was taken, the defocus amount changed by the chromatic aberration is compensated by the excitation of the objective lens or the sample height, and the same defocus as before the acceleration voltage change West,
In this state, the Ronchigram image 2 is taken by the imaging means,
The Ronchigram image 1 and the Ronchigram image 2 are used to cross-correlate near the center of the aberration, and the deflector above the objective lens deflects the electron beam in the direction of the cross-correlation peak. Take a picture and repeat the above cross-correlation process until the cross-correlation peak position is at the center
A method for determining a Ronchigram center.

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