JP2009134991A - Method and program for setting automatic focus and automatic astigmatism - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently perform focus correction and astigmatism correction in a short time. <P>SOLUTION: The method for setting automatic focus and automatic astigmatism includes a first process to scan a sample and acquire an electron microscope image by an electron detector, a second process to find an image integrated value in a prescribed angle direction for each angle with respect to the acquired image, a third process to find the standard deviation of the acquired image integrated values, a fourth process to change a focal distance by changing excitation of an objective lens by a prescribed amount, a fifth process to check whether the designated number of image sheets is acquired or not, a sixth process returning to the first step when designated number of image sheets is not acquired and when the designated number of image sheets is acquired, plotting the maximal value of the image integrated values versus focal conditions, and a seventh process to find an ellipse having the maximal value for each angle as a polar coordinate from the plots of the standard deviations obtained in the sixth process and to find the focus and astigmatism correction values from the ellipse. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an automatic focus / automatic astigmatism setting method and an automatic focus / automatic astigmatism setting program, and further, an automatic focus / automatic astigmatism setting method and an automatic focus / It relates to an automatic astigmatism setting program.

  A beam tilt method is well known as a first method of automatic focusing in a TEM (transmission electron microscope). This method uses the basic principle of TEM that an electron beam that passes through one point of the sample (object surface) is converged to one point on the image plane by a lens within an approximate range that does not consider aberration. In this method, an electron beam incident on a sample is tilted by ± K degrees, and a TEM image is acquired under each condition. If the image is shifted, it is determined that the normal focus condition is not satisfied.

  Since the shift amount of the image and the shift amount from the normal focus condition are approximately proportional to each other, the two TEM images are convolved to calculate the shift amount numerically, and the shift amount from the positive focus condition of the objective lens. And feed back to the TEM so that the amount of deviation is zero. This method is also applied to astigmatism correction. That is, when the electron beam incident on the sample is tilted, the anisotropy of deviation from the normal focus condition can be derived by changing the tilt direction.

  As a second method, a method is known in which an amorphous amorphous material is selected as a sample, and the amount of deviation from the normal focus condition is estimated from the TEM pattern. This utilizes the phase contrast transfer function of the TEM lens system. The transfer function is mainly determined by the spherical aberration coefficient, the acceleration voltage, and the amount of deviation from the normal focus condition of the objective lens.

  The FFT pattern of the TEM image of the amorphous material has a one-to-one correspondence with the phase contrast transfer function. If the acceleration voltage and the spherical aberration coefficient are known from the design specifications, the amount of deviation from the normal focus condition is determined from the shape. The anisotropy of the FFT pattern reflects the direction and amount of astigmatism, and astigmatism correction is performed using this.

  The third method is to repeat an operation of acquiring an image by changing a minute amount of excitation of an objective lens in a SEM (scanning electron microscope) and an STEM (scanning transmission electron microscope) and digitizing the contrast from the acquired image. is there. This is repeated until the contrast is maximized. The condition that maximizes the contrast is the normal focus condition, and is fed back to the SEM and STEM. For astigmatism correction, an image is acquired by changing the excitation of the astigmatism correction coil and a condition for maximizing the contrast is searched.

  As a conventional device of this type, a two-dimensional intensity distribution of an image obtained by an electron microscope is integrated along a certain one-dimensional direction, and the integrated function is subjected to a one-dimensional Fourier transform, and after the conversion As a function of defocus amount or astigmatism amount, the value obtained by integrating these functions is the extreme value of the curve formed by the function, or the position where it is almost symmetric. There is a known technique (see, for example, Patent Document 1).

In addition, a magnified image of the sample obtained by irradiating the sample with an electron beam while changing the excitation current of the objective lens and the astigmatism correction coil is photographed with an imaging device comprising an optical lens and an imaging device, and the image is sharpened with an arithmetic device. A technique is known in which the degree is calculated and the astigmatism correction value and the focus correction value are obtained from the obtained image sharpness (see, for example, Patent Document 2).
JP-A-3-194839 (page 2, lower left column, line 19 to page 3, upper right column, line 16, FIG. 1) Japanese Patent Laying-Open No. 2005-108567 (paragraphs 0030 to 0077, FIGS. 3 to 8)

  In the first method in TEM, an image is cut by an objective aperture. In TEM, a diaphragm is placed directly below the objective lens in order to increase the contrast of a normal image. This diaphragm cuts the electron beam scattered by the sample. The smaller the diaphragm, the better the contrast.

  The objective aperture is placed on the back focal plane of the objective lens. According to the first method, when the electron beam is tilted, the trajectory of the electron beam on the back focal plane moves. Thereby, the electron beam which was not scattered by the sample may be cut by the objective aperture. This happens as the aperture is reduced. If the electron beam is cut, a TEM image cannot be acquired, and there is a problem that it is impossible to determine the normal focus condition.

The second method has a problem that an amorphous sample must be selected as the sample.
Consider the case of the third method. In a method generally used in SEM and STEM, an image must be repeatedly acquired in order to perform astigmatism correction again after acquiring an image by repetition to obtain the normal focus. Since the astigmatism correction usually needs to be performed in two directions, the x direction and the y direction, it has to be repeated a total of three times in combination with the operation for obtaining the normal focus, and there is a problem that it takes time.

  The present invention has been made in view of such problems, and an automatic focus / auto astigmatism setting method and an auto focus / auto astigmatism setting program capable of efficiently performing focus correction and astigmatism correction in a short time. The purpose is to provide.

(1) According to the first aspect of the present invention, a first step of scanning a sample and acquiring an electron microscope image with an electron detector, and an image integrated value for a predetermined angle direction with respect to the acquired image for each angle. A second step for obtaining the standard deviation of the obtained image integrated value, a fourth step for changing the focal length by changing the excitation of the objective lens by a predetermined amount, and a specified number of images. A fifth step of checking whether or not the image has been acquired. If the specified number of images has not been acquired, the process returns to the first step. If the specified number of images has been acquired, A sixth step of plotting the maximum value against the focus condition, and an ellipse having the maximum value for each angle as polar coordinates are obtained from the standard deviation plot obtained in the sixth step. A seventh step of obtaining a point correction value. Is the fact characterized.
(2) In the invention according to claim 2, the seventh step stores in advance in which direction and how much the ellipse changes according to the amount of astigmatism deviation, and the major axis and minor axis of the ellipse The astigmatic difference is obtained by measuring the astigmatism difference, the astigmatism correction value is obtained based on the stored data, and a half of the sum of the major axis and the minor axis is used as the focus correction value.
(3) The invention according to claim 3 is a first step of scanning a sample and acquiring an electron microscope image with an electron detector, and an image integrated value for a predetermined angle direction with respect to the acquired image for each angle. A second step for obtaining the standard deviation of the obtained image integrated value, a fourth step for changing the focal length by changing the excitation of the objective lens by a predetermined amount, and a specified number of images. A fifth step of checking whether or not the image has been acquired. If the specified number of images has not been acquired, the process returns to the first step. If the specified number of images has been acquired, A sixth step of plotting the maximum value against the focus condition, and an ellipse having the maximum value for each angle as polar coordinates are obtained from the standard deviation plot obtained in the sixth step. A seventh step for obtaining a point correction value; And executes in data.

(1) According to the first aspect of the present invention, if a predetermined number of sample images are acquired by changing the excitation of the objective lens, the predetermined number of images are arithmetically processed to obtain a focus correction value and a non-focus value. A point correction value can be obtained, and focus correction and astigmatism correction can be efficiently performed in a short time.
(2) According to the invention described in claim 2, the focus correction value and the astigmatism correction value can be obtained from the ellipse image expressed in polar coordinates.
(3) According to the third aspect of the present invention, if a predetermined number of sample images are acquired by changing the excitation of the objective lens, the predetermined number of images are arithmetically processed to obtain a focus correction value and a non-correction value. A point correction value can be obtained, and focus correction and astigmatism correction can be efficiently performed in a short time.

  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 for automatic focusing and astigmatization of a STEM image. In the figure, 10 is a STEM (scanning transmission electron microscope). In the STEM 10, 1 is an electron beam source that emits electrons, and EB is an electron beam emitted from the electron beam source 1. Reference numeral 2 denotes a first electron optical system as an irradiation optical system for focusing the electron beam EB emitted from the electron beam source 1, and 3 denotes a sample holder for holding a sample 3a.

  4 is a second electron optical system as an imaging optical system that focuses the electron beam transmitted through the sample 3a, and 5 is a multipole for correcting the astigmatism of the electron beam focused by the second electron optical system 4. A coil system 6 is disposed on the imaging surface of the transmission electron beam, an electron beam detector for converting the electron beam into an electric signal, and 7 receives the output of the electron beam detector 6 and displays the transmission image on the display 7a. A computer 7b for displaying is a program for executing the present invention stored in the computer 7. The computer 7 controls the second electron optical system 4 and the multipole coil 5. The second electron optical system 4 includes an objective lens. Here, for example, a television camera is used as the electron beam detector 6. The operation of the present invention will be described using the apparatus configured as described above.

  FIG. 2 is a flowchart for automatic focusing and astigmatization of a STEM image. First, the computer 7 acquires a STEM image from the signal detected by the electron beam detector 6 (S1). Then, the computer 7 performs noise removal processing and edge enhancement processing on the acquired image. As the noise removal processing, for example, a median filter is used. As the edge enhancement processing, for example, a differential filter or a Laplacian filter is used.

  Next, the image subjected to noise removal and edge enhancement is stored in a memory (not shown) in the computer 7. Next, the image stored in the memory is read out, and image information is accumulated in the directions shown in FIG. 3 to obtain one-dimensional information (S2). FIG. 3 is an explanatory diagram of the integration direction of image information. Four directions from circle 1 to circle 4 are shown. Circle 1 indicates the integration direction when the angle is 0 degrees with respect to the X and Y directions of the image, Circle 2 indicates the integration direction when the angle is 45 degrees with respect to the X and Y directions of the image, and Circle 3 indicates the X direction of the image. The circle 4 indicates the integration direction when the angle is 90 degrees with respect to the Y direction, and the circle 4 indicates the integration direction when the angle is 135 degrees with respect to the X and Y directions of the image. A indicates the integrated state (projection) of the image. In the figure, ● indicates that it corresponds to TEM.

  In step S2, an image projection (integrated image) is obtained for each direction described above. Next, the computer 7 calculates a standard deviation for the obtained projection (S3). When the standard deviation is obtained for each direction, the computer 7 sends a control signal to the second electron optical system 4 to change the value of the current passed through the objective lens. This corresponds to changing the focal length of the image (S4). This excitation current changing step corresponds to the focal amount change Δf.

  Next, the computer 7 checks whether or not a specified number (for example, 10) of images has been acquired (S5). If the specified number has not been acquired, the sequence returns to step S1, and the sequence of STEM image acquisition → image projection acquisition → standard deviation calculation → focal length change is repeated. As a result, the standard deviation for the focal length change amount Δf is stored in the memory of the computer 7 for each angle.

  When the designated number of images is acquired in step S5, the computer 7 plots the maximum value of the projection against the focus condition (S6). FIG. 4 is an explanatory diagram when the standard deviation-focus condition plot for each projection obtained in this way is obtained for each angle. The vertical axis represents the standard deviation σn, and the horizontal axis represents the focus condition (focal length change amount) Δf of the second optical system. That is, the horizontal axis represents the standard deviation σn obtained by changing the focal point from a certain value by step Δf. Here, a case where 10 images are acquired is shown. FIG. 4 shows the standard deviation when the angles are 0 degree, 45 degrees, 90 degrees, and 135 degrees. This standard deviation image is displayed on the display 7a.

  As shown in FIG. 4, when the standard deviation is obtained with respect to the focus condition, the maximum value of the standard deviation in the case of each angle is obtained as polar coordinates corresponding to the amplitude and angle, and the obtained figure is as shown in FIG. Becomes an ellipse. FIG. 5 shows a diagram in which the peak position of the standard deviation-focus condition plot is plotted in polar coordinates with respect to the direction of each projection. In the figure, ● is the plot value. Since the maximum value changes depending on the direction and size of the astigmatism, when this peak position is displayed in polar coordinates, an ellipse as shown in FIG. 5 is obtained. This ellipse is displayed on the display 7a. If there is no astigmatism, it is not an ellipse but a perfect circle.

  The focus and astigmatism correction values are obtained from the ellipse thus obtained and set (S7). Hereinafter, the calculation method will be described. In this case, it is stored in advance in which direction and how much the ellipse changes according to the amount of astigmatism deviation. Next, the major axis and minor axis of the ellipse are measured to determine the astigmatic difference, and the astigmatism correction value is determined based on the stored data.

Here, the difference (ab) between the major axis length a and the minor axis length b of the ellipse corresponds to an astigmatic difference. The computer 7 sets the second optical system 4 under a condition corresponding to the center of this astigmatism. Astigmatism can now be corrected. The focus correction is obtained as follows. When the longest distance (major axis) from the origin in FIG. 5 is Lmax and the shortest distance (minor axis) is Lmin, the optimum focus is
(Lmax + Lmin) / 2
Is obtained as a value corresponding to. Specifically, astigmatism correction is performed by applying a correction signal from the computer 7 to the multipole coil system 5 based on the acquired correction data, and focus correction is performed from the computer 7 to the objective of the second electron optical system 4. This is done by correcting the excitation of the lens.

  As described above, according to the present invention, if a predetermined number of sample images are acquired by changing the excitation of the objective lens, the predetermined number of images are arithmetically processed to perform the focus correction value and the astigmatism correction value. Therefore, focus correction and astigmatism correction can be performed efficiently in a short time.

Further, according to the present invention, the focus correction value and the astigmatism correction value can be obtained from an elliptic image represented by polar coordinates.
According to the present invention, the above-described series of steps can be executed by the program 7b of the computer 7.

  According to the present invention, it is possible to provide an automatic focus / automatic astigmatism setting method for solving all the problems of the first method to the third method of the conventional method described above. That is, according to the present invention, the electron beam information is not lost because it is not necessary to tilt the electron beam as in the first method, and it is not necessary to use an amorphous sample as in the second method. Further, unlike the third method, a plurality of data acquisition steps are not required.

It is a figure which shows the structural example for the automatic focusing and astigmatism of a STEM image. It is a flowchart of automatic focusing and astigmatization of a STEM image. It is explanatory drawing of the integration direction of image information. It is explanatory drawing of the standard deviation-focus condition plot with respect to each projection. It is the figure which plotted the peak position of the standard deviation-focus condition plot in polar coordinates with respect to the direction of each projection.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electron beam source 2 1st electron optical system 3 Sample holder 3a Sample 4 2nd electron optical system 5 Multipole coil diameter for correcting astigmatism 6 Electron beam detector 7 Computer 7a Display 7b Program

Claims (3)

A first step of scanning a sample and obtaining an electron microscope image with an electron detector;
A second step of obtaining, for each angle, an image integration value for a predetermined angle direction with respect to the acquired image;
A third step of obtaining a standard deviation of the obtained image integrated value;
A fourth step of changing the focal length by changing the excitation of the objective lens by a predetermined amount;
A fifth step of checking whether or not a specified number of images has been acquired;
If the specified number of images has not been acquired, the process returns to the first step. If the specified number of images has been acquired, the maximum value of the integrated image value is plotted against the focus condition. Process,
A seventh step of obtaining an ellipse having the maximum value for each angle as a polar coordinate from the standard deviation plot obtained in the sixth step, and obtaining a focus and astigmatism correction value from the ellipse;
An automatic focus / automatic astigmatism setting method characterized by comprising:   The seventh step stores in advance how much the ellipse changes in which direction according to the amount of astigmatism shift, measures the major axis and minor axis of the ellipse, finds the astigmatic difference, 2. The automatic focus / auto astigmatism correction method according to claim 1, wherein an astigmatism correction value is obtained based on the stored data, and ½ of the sum of the major axis and the minor axis is used as the focus correction value. A first step of scanning a sample and obtaining an electron microscope image with an electron detector;
A second step of obtaining, for each angle, an image integration value for a predetermined angle direction with respect to the acquired image;
A third step of obtaining a standard deviation of the obtained image integrated value;
A fourth step of changing the focal length by changing the excitation of the objective lens by a predetermined amount;
A fifth step of checking whether or not a specified number of images has been acquired;
If the specified number of images has not been acquired, the process returns to the first step. If the specified number of images has been acquired, the maximum value of the integrated image value is plotted against the focus condition. Process,
A seventh step of obtaining an ellipse having the maximum value for each angle as a polar coordinate from the standard deviation plot obtained in the sixth step, and obtaining a focus and astigmatism correction value from the ellipse;
An automatic focus and automatic astigmatism correction program characterized by being executed by a computer.