JP3112541B2 - Astigmatism correction method for electron beam device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting astigmatism in an electron beam apparatus, such as a scanning electron microscope, which can automatically correct astigmatism.

[0002]

2. Description of the Related Art In an electron beam apparatus such as a scanning electron microscope, astigmatism of an electron beam must be corrected before observation of an image. In this case, first, the focusing operation of the electron beam is performed to focus on the minimum confusion circle position, and then, among the X and Y astigmatism correction coils, first, a plurality of astigmatism correction coils are applied to the X direction astigmatism correction coil. A correction value is given, an electron beam is scanned for each astigmatism correction value, and for example, a secondary electron signal obtained according to this scanning is detected. By such an operation, a curve as shown in FIG. 1 is obtained. The horizontal axis in FIG. 1 is the astigmatism correction value in the X or Y direction, and the vertical axis is the signal intensity. The astigmatism correction value at the peak position of this curve is the best value, and this value is set in the X direction astigmatism correction coil. Next, a plurality of astigmatism correction values are given to the Y direction astigmatism correction coil, and the best astigmatism correction value is set to the Y direction astigmatism correction coil as in the X direction.

[0003]

In the above-described conventional astigmatism correction operation, the electron beam scanning must be performed each time the astigmatism correction in the X and Y2 directions, and the time required for the astigmatism correction operation becomes long. is there.

[0004] The present invention has been made in view of such a point, and an object of the present invention is to provide a method for correcting astigmatism in an electron beam apparatus capable of accurately correcting astigmatism in the X and Y directions in a short time. It is to realize.

[0005]

According to the present invention, there is provided an astigmatism correction method for an electron beam apparatus, comprising: an objective lens for narrowly focusing an electron beam irradiated on a sample; An electron beam apparatus comprising: a deflecting unit for two-dimensionally scanning in the Y direction; and an astigmatism correction device in the XY directions arranged in the electron beam path.
Scanning the electron beam two-dimensionally on the sample, integrating a signal obtained by the scanning, performing the electron beam scanning many times while changing the excitation intensity of the objective lens, and adjusting the excitation intensity of the objective lens. A step of obtaining a shift amount D of the focus between the center of the two peaks of the change curve of the integral value due to the change, and a plurality of non-focus correction devices supplied to the astigmatism correction device stored in advance according to the shift amount D; Reading out the astigmatism correction value corresponding to the obtained deviation amount D from the point correction values, supplying each astigmatism correction value to the astigmatism correction device, and scanning the electron beam each time; the step of integrating the resulting signals, the astigmatic correction value when the peak of the variation curve of the integral value with changes in astigmatism correction value characterized by consisting of the step of setting the astigmatism corrector There.

[0006]

The astigmatism correction method in the electron beam apparatus according to the present invention scans an electron beam two-dimensionally on a sample, integrates a signal obtained by this scanning, and scans the electron beam with an objective lens. This is performed a number of times while changing the excitation intensity, and the shift amount D of the focus between the center of the two peaks and the peak of the integrated value change curve due to the change in the excitation intensity of the objective lens is obtained. Among the plurality of astigmatism correction values supplied to the astigmatism correction device stored in advance, the astigmatism correction value corresponding to the obtained shift amount D is read, and each astigmatism correction value is sent to the astigmatism correction device. supplied by scanning each time the electron beam, integrating the signal obtained by the scanning, the astigmatic correction value when the peak of the variation curve of the integral value with changes in astigmatism correction value astigmatism corrector Set to.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 2 shows a scanning electron microscope for implementing the astigmatism correction method according to the present invention, and 1 is an electron gun of the scanning electron microscope. The electron beam EB generated from the electron gun 1 is narrowly focused on the sample 4 by the objective lens 2 and the objective lens 3. Reference numeral 5 denotes a deflection coil, which scans the sample 4 with the electron beam EB according to a scanning signal from the scanning signal generating circuit 6. 7 is an astigmatism correction coil in the X direction, 8
Is a Y-direction astigmatism correction coil. The X-direction astigmatism correction coil 7 is driven by a coil driving unit 9. The X-direction coil driving unit 9 has an X-direction astigmatism correction coil set in a register 10. The value data is supplied via the DA converter 12. The Y-direction astigmatism correction coil 8 is driven by the coil drive unit 13, and the Y-direction astigmatism correction value data set in the register 14 is supplied to the Y-direction coil drive unit 13 via the DA converter 15. Is done. The objective lens 3 is a driving unit 1
The objective lens current value data set in the register 17 is supplied to the drive unit 16 via the DA converter 18. Reference numeral 19 denotes a detector for detecting secondary electrons generated by irradiating the sample 4 with an electron beam. A detection signal of the detector 19 is supplied to an integrator 22 via a filter circuit 20 and an absolute value circuit 21. Integrator 22
Is supplied to the memory 24 via the AD converter 23 and stored. The CPU 26 creates a signal intensity distribution from the signals stored in the memory 24 and detects a peak value. Further, the CPU 26 includes a scanning signal generation circuit 6, an integrator 2
2 and the like, read the value of the data memory 27, set the astigmatism correction value data in the registers 11 and 13, and set the objective lens current value data in the register 17. Next, the operation of such a configuration will be described.

When a secondary electron image of a normal sample 4 is obtained, the electron beam E is emitted by the objective lens 2 and the objective lens 3.
B is narrowly focused on the sample 4, a signal for scanning the electron beam two-dimensionally is supplied from the scanning signal generation circuit 6 to the deflection coil 5, and the detection signal from the secondary electron detector 19 is shown. Not supply to the cathode ray tube. Now, the operation of correcting the astigmatism of the electron beam will be described. First,
The excitation intensity of the objective lens 3 is changed in minute steps, and a predetermined region on the sample 4 is scanned with the electron beam for each excitation intensity. In this case, the CPU 26 changes the objective lens current value set in the register 17 in minute steps, and the CPU 26 controls the scanning signal generation circuit 6 so that one predetermined two-dimensional scan is performed for each objective lens current value. .

Secondary electrons generated by irradiating the sample 4 with the electron beam EB are detected by a detector 19, and are transmitted through a filter circuit 20 and an absolute value circuit 21 to an integrator 22.
, But the integrator 22 provides a single 2
Integrate the detection signal during the dimensional scan. After the one-dimensional scanning of the electron beam is completed, the value integrated by the integrator 22 is supplied to the memory 24, and the objective lens 3 at that time is supplied to the memory 24.
Is stored in accordance with the excitation intensity of. Such two-dimensional scanning of the electron beam and integration of the secondary electron detection signal are performed for each excitation intensity of the objective lens 3, and each integrated value is stored in the memory 24. FIG. 3 shows a signal intensity distribution created by the CPU 26 from the signals stored in the memory 24. In FIG. 3, the horizontal axis represents the excitation intensity of the objective lens, and the vertical axis represents the integrated value. The distribution in FIG. 3 is a case where the electron beam has no astigmatism, and the electron beam is accurately focused at the excitation intensity of the objective lens at the peak position. In other words, the current value of the objective lens at the time of this peak is stored in the register 1
By setting to 7, focusing can be performed.

When the electron beam has a stigma, the signal intensity distribution generated by the CPU 26 has two peaks (P1, P2) as shown in FIG. This peak P
The position at which the intermediate position P0 between 1 and P2 is in focus,
That is, it is known that this is the position where the circle of least confusion is formed. The minimum confusion circle position P0 and the peaks P1, P
2 corresponds to the amount of deviation of the astigmatism of the electron beam. If the deviation amount D of the astigmatism is large, the amount of the astigmatism is large, and if it is small, the astigmatism is small. The CPU 26 detects the amount D according to the size of the astigmatism. By the way, to correct the size of a certain astigmatism, X,
The astigmatism correction value that must be supplied to the astigmatism correction coils 7 and 8 in the Y direction is naturally determined. Data memory 27
Stores a combination of the astigmatism correction amounts to be supplied to the X and Y astigmatism correction coils 7 and 8 for correcting the astigmatism in accordance with the amount D of each astigmatism. In this case, a plurality of combinations of X and Y are stored for each astigmatism amount D.

When the CPU 26 detects the astigmatism shift amount D, the CPU 26 reads a plurality of astigmatism correction amounts corresponding to the D from the data memory 27, and sequentially registers them in the register 1.
Set to 1,13. Correction currents are supplied from the drive units 9 and 10 to the astigmatism correction coils 7 and 8 in accordance with the astigmatism correction amounts set in the registers 11 and 13. A plurality of combinations of the astigmatism correction amounts are sequentially supplied to the astigmatism correction coils 7 and 8. Each time the combination of the astigmatism correction amounts is supplied to the coils 7 and 8, the scanning signal generation circuit 6 Thus, a scanning signal for two-dimensionally scanning the sample 4 is supplied to the deflection coil 5, and a predetermined area on the sample is scanned by the electron beam. Secondary electrons generated by this scanning are detected by the detector 19, and the detection signal is integrated by the integrator 22. The integral value is supplied to the memory 24 for each two-dimensional scan, and the integral value is stored corresponding to the combination of the X and Y astigmatism correction amounts.

As a result, after all the combinations of the X and Y astigmatism correction amounts are supplied to the astigmatism correction coils 7 and 8, the CP
U26 obtains a signal intensity distribution as shown in FIG. In FIG. 5, the horizontal axis represents each combination of the astigmatism correction amounts in the XY directions, and the vertical axis represents the signal intensity. X at the peak position of this distribution
The amount of astigmatism correction in the Y direction is the optimum amount of astigmatism correction, and this value is set in the registers 11 and 13. Thereafter, if a normal scanning image is observed, scanning on the sample can always be performed with the optimally astigmatically corrected electron beam, and an image with high resolution can be obtained. In this manner, the correction of the astigmatism in the XY2 directions can be performed by one astigmatism correction operation.

Although the embodiment of the present invention has been described in detail, the present invention is not limited to this embodiment. For example, secondary electrons are detected, but reflected electrons and the like may be detected. Further, the data memory 27 stores the combination of the astigmatism correction values in the XY directions corresponding to the astigmatism deviation amount D. The deviation amount D is calculated from the astigmatism deviation amount D by a predetermined calculation formula. May be sequentially determined. Further, the electron beam was used to scan the sample two-dimensionally in order to obtain the shift amount D. For example, as shown in FIG. 6, one line (A, B) of each of X and Y on the sample 4 was scanned. Alternatively, the shift amount D may be obtained based on a detection signal based on the scanning. In this case, as shown in FIG. 7, a curve a based on the line scanning in the X direction and a curve b based on the line scanning in the Y direction are obtained as the detection signal distribution, and each peak position of the curves a and b is obtained. Is the minimum confusion circle position, and the amount between the intermediate position and each peak is the astigmatism shift amount. Although the excitation intensity of the objective lens is changed during the autofocus operation, the excitation intensity of the auxiliary lens may be changed by using an auxiliary lens of the objective lens.

[0014]

As described above, the astigmatism correction method in the electron beam apparatus according to the present invention scans an electron beam two-dimensionally on a sample, integrates a signal obtained by this scanning, and This electron beam scanning is performed a number of times while changing the excitation intensity of the objective lens, and the focus shift amount D between the center of the two peaks and the peak of the change curve of the integral value due to the change in the excitation intensity of the objective lens. Is obtained, and among the plurality of astigmatism correction values to be supplied to the astigmatism correction device stored in advance in accordance with the shift amount D, the astigmatism correction value corresponding to the calculated shift amount D is read, and each astigmatism correction is performed. The value is supplied to the astigmatism correction device, the electron beam is scanned each time, the signal obtained by this scanning is integrated, and the astigmatism at the peak of the change curve of the integral value accompanying the change of the astigmatism correction value is obtained. Correction value astigmatism Since so as to set the positive device, it is possible to perform accurate X, correction of astigmatism in the Y direction in a short time.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a change in signal intensity with respect to an astigmatism correction value.

FIG. 2 shows a scanning electron microscope for implementing the method according to the invention.

FIG. 3 is a diagram showing a change in an integrated value of a detection signal with respect to an excitation intensity of an objective lens.

FIG. 4 is a diagram showing a change in an integrated value of a detection signal with respect to an excitation intensity of an objective lens.

FIG. 5 is a diagram illustrating a combination of X and Y astigmatism correction amounts and a change in detection signal intensity.

FIG. 6 is a diagram showing a state of scanning of an electron beam on a sample in another embodiment of the present invention.

FIG. 7 is a diagram showing a change curve of a signal obtained by the scanning in FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Electron gun 2 ... Objective lens 3 ... Objective lens 4 ... Sample 5 ... Deflection coil 6 ... Scanning signal generation circuit 7, 8 ... Astigmatism correction coil 9, 13, 16 ... Driving unit 10, 14, 17 ... Register 12, 15, 18 DA converter 19 Secondary electron detector 22 Integrator 24 Memory 26 CPU 27 Data memory

Claims (1)

(57) [Claims] 1. A focusing lens for narrowly focusing an electron beam irradiated on a sample, a deflecting unit for scanning the electron beam two-dimensionally in an X direction and a Y direction on the sample, and an electron beam path. Scanning an electron beam two-dimensionally on a sample and integrating a signal obtained by the scanning in an electron beam apparatus having an XY direction astigmatism correction device disposed in Is performed a number of times while changing the excitation intensity of the objective lens, the amount of focus shift between the center of the two peaks and the peak of the change curve of the integral value when there is a non-point due to the change in the excitation intensity of the objective lens D. a step of reading out the astigmatism correction value corresponding to the calculated shift amount D from among a plurality of astigmatism correction values to be supplied to the astigmatism correction device stored in advance according to the shift amount D. Step of supplying each astigmatism correction value to the astigmatism correction device, scanning the electron beam each time, and integrating a signal obtained by this scanning, with the change of the astigmatism correction value A method for correcting astigmatism in an electron beam device, comprising: setting an astigmatism correction value at a peak of a change curve of an integral value in an astigmatism correction device.