JPH08264147A - Electron beam adjusting method in scanning electron microscope - Google Patents

Abstract

PURPOSE: To provide an electron beam adjusting method in a scanning electron microscope which can carry out automatic focusing at high precision even if an axis divergence occurs in an objective lens. CONSTITUTION: A unit 36 to detect an image moving direction and the moving degree detects the moving direction and the moving degree of an image based on integrated values for every divided region and the detected signals are supplied to a driving circuit 33 for correcting coils 31, 32. As a result, even if there are axis divergence of an objective lens 4 and position shift of an image owing to the focus point alteration of a step type electron beam, the shift of the image can be compensated by correction signals to the correcting coils 31, 32.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of adjusting an electron beam in a scanning electron microscope having an autofocus function for automatically focusing.

[0002]

2. Description of the Related Art FIG. 1 shows an example of a conventional scanning electron microscope having an autofocus function. Reference numeral 1 is an electron beam generated from an electron gun (not shown) and accelerated.
2 and 3 are two-stage deflection coils, and two-stage deflection coils 2 and 3
Each of these includes horizontal and vertical deflection coils. Reference numeral 4 is an objective lens, and the sample 5 is irradiated with the electron beam finely focused by the objective lens 4. 6 is generated by irradiating the sample 5 with an electron beam, for example, 2
It is a detector for detecting secondary electrons.

A horizontal scanning signal is supplied to the horizontal deflection coils of the two-stage deflection coils 2 and 3 from a horizontal scanning signal generating circuit 7 through a driving circuit 8, and a vertical scanning signal is generated to the vertical deflection coils. A vertical scanning signal is supplied from the circuit 9 via the drive circuit 10. Speed (cycle) of scanning signals from the horizontal scanning signal generation circuit 7 and the vertical scanning signal generation circuit 9
Are controlled by a control circuit 11 such as a computer. The signal detected by the detector 6 is the amplifier 1
The signal is supplied to the cathode ray tube 13 and the filter circuit 14 to which the horizontal and vertical scanning signals are supplied via 2. The output signal of the filter circuit 14 is supplied to the integrating circuit 16 via the absolute value circuit 15 and integrated by the integrating circuit 16. The integrated value of the integrating circuit 16 is converted into a digital signal by the AD converter 17 and stored in the signal intensity distribution memory 19 in the control circuit 11.

The control circuit 11 has a maximum value detecting unit 20 for detecting the maximum value of a large number of integrated values stored in the signal intensity distribution memory 19. Further, the control circuit 11
Has an objective lens value setting data memory 21 and an auxiliary coil value setting data memory 22. The value of the objective lens value setting data memory 21 is supplied to the objective lens drive circuit 24 via the DA converter 23. Further, the value of the auxiliary coil value setting data memory 22 is supplied to the drive circuit 27 of the auxiliary coil 26 via the DA converter 25. Reference numeral 28 is an objective lens value conversion unit, and 29 is an autofocus unit. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the control circuit 11 controls the horizontal scanning signal generating circuit 7 and the vertical scanning signal generating circuit 9 so as to obtain a desired scanning speed, and as a result, a desired deflection signal is generated. The electron beam 1 supplied to the deflection coils 2 and 3 is deflected by the deflection coils 2 and 3, and scans a desired region of the sample 5. Secondary electrons generated by the irradiation of the sample 5 with the electron beam are detected by the detector 6,
The detection signal is amplified by the amplifier 12 and then supplied to the cathode ray tube 13 to which the scanning signal is supplied, so that the secondary electron image of the sample is displayed on the cathode ray tube 13.

Next, the autofocus operation will be described with reference to FIG.
The operation curve of the auxiliary coil 26 in (a) and the operation curve of the objective lens 4 in FIG. 2 (b) will be described. 2A and 2B, the horizontal axis represents time and the vertical axis represents excitation intensity (equal to the distance between the sample surface and the lens). First, the objective lens value setting data memory 21 is shown in FIG.
The initial value ΔZ shown in is set, and the objective lens drive circuit 24 is operated based on this initial value to excite the objective lens 4. Next, the autofocus start unit 29 is operated to change the value of the auxiliary coil value setting data memory 22 to the value shown in FIG.
Change as shown in (a). In FIG. 2A, the change of the set value is shown in a straight line for the sake of convenience.
It is changed stepwise as shown in FIG.

Each time this stepwise change is made, the deflection coils 2 and 3 are supplied with a scanning signal for scanning the desired region on the sample 5 once. Secondary electrons generated by the irradiation of the sample 5 with the electron beam are detected by the detector 6. The detection signal is amplified by the amplifier 12, and after the specific frequency component is cut by the filter circuit 14,
The negative signal is inverted in the absolute value circuit 15.

The output of the absolute value circuit 15 is supplied to the integration circuit 16 to integrate the signal. This integration is executed during one two-dimensional scanning of the desired area on the sample, and after the scanning is completed, the integrated value is sent to the signal intensity distribution memory 19 in the control circuit 11 via the AD converter 17. Will be remembered. When such an integration operation is performed for each stepwise excitation change of the auxiliary coil 26 and from + A to −A,
The distribution shown in FIG. 3B will be stored in the memory 19.

Maximum value detection unit 20 in control circuit 11
Detects the maximum value of the distribution shown in FIG. 3B, and the excitation intensity to the auxiliary coil 26 at that time is determined by the objective lens value conversion unit 2
Supply to 8. The electron beam is in focus during this maximum excitation. As a result, the value of the objective lens value setting data memory 21 is a value obtained by adding the excitation intensity (amount of deviation between the initial value ΔZ and the focus position) ΔZ1 of the auxiliary coil when the initial setting value ΔZ is in focus. Becomes After the excitation of the objective lens 4 is set to ΔZ + ΔZ1, the above autofocus operation is executed again, and the deviation amount ΔZ2 between ΔZ + ΔZ1 and the in-focus value at that time is set in the auxiliary coil value setting data memory 29. The excitation value ΔZ1 of the objective lens and the excitation value ΔZ2 of the auxiliary coil that are set are shown in FIGS. 2 (a) and 2 (b). When the setting of ΔZ1 and ΔZ2 is completed, the autofocus operation ends.

[0010]

In the above-described autofocus, the axes of the objective lens 4 and the auxiliary lens 26 are mechanically and electrically corrected, and the optical axes are aligned. However, when the axis of the objective lens 4 is originally deviated, there is a problem that the image moves with the change in focus when the focus changes. That is, when the excitation intensity of the objective lens or the auxiliary lens 26 is changed, the scanning area of the electron beam on the sample is changed, and signals from different areas are detected and integrated. Therefore, the distribution shown in FIG. 3B is based on signals from different sample areas, which deteriorates the accuracy of the autofocus operation.

The present invention has been made in view of the above points, and an object thereof is to obtain an axial displacement of an objective lens,
It is to realize an electron beam adjusting method in a scanning electron microscope capable of performing operations such as autofocus with high accuracy.

[0012]

According to a method of adjusting an electron beam in a scanning electron microscope according to the present invention, an electron beam is focused finely on a sample, and the electron beam is focused on the sample.
With a scanning electron microscope that scans dimensionally, detects the signal obtained from the sample, and obtains a scanned image of the sample based on the detected signal, the focus of the electron beam is changed stepwise,
Based on the signal detected according to this focus change, in the electron beam adjusting method for focusing the electron beam, etc., the scanning area of the electron beam on the sample is divided into a plurality of areas and The signals obtained from the respective divided areas are respectively integrated, the moving direction and the moving amount of the image accompanying the focus change of the electron beam are obtained based on the integration result, and when the focus of the electron beam is changed stepwise again, this The feature is that the electron beam is deflected based on the obtained moving direction and moving amount of the image to eliminate the moving of the image.

[0013]

According to the method of adjusting the electron beam in the scanning electron microscope according to the present invention, the scanning region of the electron beam on the sample is divided into a plurality of regions, and the signals obtained from the respective divided regions are integrated for each change in the focal point of the electron beam. Then, the moving direction and the moving amount of the image due to the change in the focus of the electron beam are obtained based on the integration result, and when the focus of the electron beam is changed stepwise again, based on the obtained moving direction and the moving amount of the image. Then, the electron beam is deflected to prevent the influence of image movement.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 4 shows a scanning electron microscope which is an embodiment of the present invention. The same or similar parts as those of the conventional apparatus of FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. In this embodiment, correction coils 31 and 32 are arranged along the optical axis of the electron beam 1. The correction coils 31 and 32 are arranged above the deflection coils 2 and 3, but may be below the deflection coils. A signal for correcting the axis of the electron beam is supplied from the drive circuit 33 to the correction coils 31 and 32.

The signal detected by the secondary electron detector 6 is subjected to various kinds of signal processing, integrated by an integrator circuit 16, AD-converted, and then supplied to the control circuit 11. The signal supplied to the control circuit 11 is supplied to the divided area integral value memory 34, and the integrated intensity is stored for each divided area. Here, the scanning of the electron beam on the sample 5 by the deflection coils 2 and 3 is performed by dividing the specific scanning region into 9 regions.

The signal stored in the divided area integral value memory 34 is supplied to the whole area addition unit 35 and the image movement direction / movement amount detection unit 36. In the all-area adding unit 35, all integrated values for each divided area are added and supplied to the signal intensity distribution memory 19. The signal intensity distribution memory 19 is a unit similar to the conventional device of FIG. 1, and the autofocus operation is performed based on the stored contents of the memory 19, as described in the conventional device of FIG.

The image moving direction and moving amount detection unit 36 detects the moving direction and moving amount of the image from the integrated value for each divided area, and the detected signal is used as a correction coil 31,
It is supplied to the drive circuit 33 of 32. The operation of such a configuration is as follows.

The observation of a normal secondary electron image and the focusing of the objective lens 4 are performed in the same manner as in the conventional apparatus of FIG. Here, the scanning of the electron beam on the sample 5 is performed by dividing the scanning region for performing the autofocus. For example, this scan area is divided into nine areas. The integrating circuit 16 integrates the signal for each of the nine divided regions. The integrated value for each divided area is supplied to and stored in the divided area integrated value memory 34 in the control circuit 11.

FIG. 5 shows the integral value and integral value ratio for each of the 9 divided regions, FIG. 5 (a) shows the integral value of the 9 divided regions, and FIG. 5 (b). , (C) show integral value ratios. The region indicated by the triangle R in this figure is a characteristic region of the sample surface, and many secondary electrons are generated from this triangular region. 5 (b) and 5 (c)
And indicate that the scanning region of the electron beam is displaced due to the change of the exciting current supplied to the auxiliary coil 26, and the triangular region R which is the characteristic region is displaced downward. That is, in the example of this figure, the image is shifted in the -Y direction, and the integral value ratio has the signal amount moved to the -Y side as the image moves. At this time, the moving direction and the moving amount of the image can be known by comparing the increase and decrease of the integral value ratio for each region.

FIGS. 6A to 6E show the auxiliary coil 2 with respect to the area divided into 9 along with the autofocus operation.
6 shows a state in which the integrated value ratio of each divided region is changed by the change of the exciting current to 6 in five stages. In this example as well, the image is displaced in the -Y direction. Focusing on the three areas A, B, and C in the central row among the nine divided areas, and plotting the integrated value ratios of these three areas for each focus change, the graph of FIG. 7 is obtained.

In FIG. 7, the vertical axis represents the integral value ratio, and the horizontal axis represents the focus changes (a) to (e). In the figure, the change curve plotted with ◯ mark is the region A, the change curve plotted with X mark is the region B, and the change curve plotted with Δ mark is the region C. Here, the maximum value of the integrated value ratio moves along with the focus change. The movement direction of the image in the control circuit 11 and the movement amount detection unit 36 detect this maximum value,
The movement amount with respect to the focus change is calculated. This Figure 6,
In FIG. 7, above and in the middle row, for ease of explanation,
Although the description has been made by paying attention to the changes in the lower three regions, in reality, the changes in the integral value ratio are similarly detected in each of the nine regions, and the moving direction and the moving amount of the image are obtained.

By the steps described above, the auxiliary coil 26
After detecting the moving direction and the moving amount of the image when the exciting current is changed, the autofocus operation is performed again when the image is moved. At this time, the drive circuit 33 applies the correction current to the correction coils 31 and 32 based on the detected moving direction and moving amount of the image for each change of the exciting current of the auxiliary coil 26, and moves in the opposite direction to the moving of the image. The electron beam is deflected by the same amount, and the axis of the electron beam is corrected so that the image does not move as a result.

The autofocus operation is performed by adding the data stored in the divided area integral value memory 34 by the all area adding unit 35 for each excitation step and supplying the added data to the signal intensity distribution memory 19. . Based on the signal strength distribution obtained in the memory 19,
The step of detecting the maximum value by the maximum value detection unit 20 and executing the autofocus based on the detected value is the same as in the conventional apparatus of FIG. A flow of such an autofocus operation is shown in FIG.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to this embodiment. For example, although secondary electrons are detected, reflected electrons may be detected. Further, although the auto focus operation has been described as an example, the present invention can be used not only in the auto focus operation but also in the auto astigmatism operation, auto contrast operation, and auto brightness operation. Furthermore, the case where the scanning area of the electron beam is divided into nine has been described.
The number of divisions may be 25, 25 or any other number.
Furthermore, although the focus of the electron beam is changed stepwise by using the auxiliary coil, the focus change signal may be superimposed on the objective lens without using the auxiliary coil.

[0025]

As described above, according to the method of adjusting the electron beam in the scanning electron microscope according to the present invention, the electron beam is finely focused on the sample, and the electron beam is two-dimensionally scanned on the sample. With a scanning electron microscope that detects the signal obtained from the sample and obtains a scanning image of the sample based on the detected signal, the focus of the electron beam is changed stepwise, and the signal detected according to this focus change Based on the above, in the electron beam adjusting method for performing electron beam focusing, etc., the scanning region of the electron beam on the sample is divided into a plurality of signals, and the signals obtained from the respective divided regions are changed for each focus change of the electron beam. The moving direction and the moving amount of the image due to the focus change of the electron beam are calculated based on the integration result, and when the focus of the electron beam is changed stepwise again, this is calculated. Based on the moving direction of the moving amount,
It is configured to deflect the electron beam.

As a result, even if the image is moved when the focal point of the electron beam is changed stepwise due to the axial displacement of the objective lens, the electron beam should be moved in the opposite direction of this movement. Can be performed and each integral value of the focus change is based on the signal from the same region of the sample. Therefore, it is possible to improve the accuracy of auto focus, auto astigmatism correction, auto contrast operation, and auto brightness operation.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional scanning electron microscope having an autofocus function.

FIG. 2 is a signal wave system diagram used for explaining an autofocus operation by the scanning electron microscope of FIG.

FIG. 3 is a signal wave system diagram used for explaining an autofocus operation by the scanning electron microscope of FIG.

FIG. 4 is a diagram showing an example of a scanning electron microscope for carrying out the method of the present invention.

FIG. 5 is a diagram showing an integral value and an integral value ratio of a signal from a sample region divided into nine parts.

FIG. 6 is a diagram showing an integral value ratio of signals from a sample region divided into nine parts.

FIG. 7 is a diagram showing an integrated value ratio graph with a change in focus.

FIG. 8 is a diagram showing a flow of an embodiment of the present invention.

[Explanation of symbols]

 1 Electron Beam 2, 3 Deflection Coil 4 Objective Lens 5 Sample 6 Detector 7 Horizontal Scan Signal Generation Circuit 8, 10, 24, 27, 35 Drive Circuit 9 Vertical Scan Signal Generation Circuit 11 Control Circuit 14 Filter Circuit 15 Absolute Value Circuit 16 Integration circuit 17 AD converter 20 Maximum value detection unit 21 Objective lens value setting data memory 22 Auxiliary coil value setting data memory 23, 25, 34 DA converter 26 Auxiliary coil 28 Objective lens value converting unit 29 Autofocus unit 31, 32 Correction Coil 33 Correction coil drive circuit 34 Divided area integral value memory 35 Full area addition unit 36 Image movement method / movement amount detection unit

Claims (1)

[Claims] 1. An electron beam is finely focused on a sample, the electron beam is two-dimensionally scanned on the sample, a signal obtained from the sample is detected, and a scan image of the sample is obtained based on the detected signal. In the scanning electron microscope, the electron beam focus is changed stepwise, and the electron beam is adjusted based on the signal detected according to this focus change. The electron beam scanning area is divided into multiple parts, and the signals obtained from each divided area are integrated for each focus change of the electron beam. Based on the integration result, the image movement direction and movement associated with the electron beam focus change Then, when the focus of the electron beam is changed stepwise again, the electron beam is deflected based on the obtained moving direction and moving amount of the image to eliminate the movement of the image. Method of adjusting electron beam in scanning electron microscope.