JPH0766773B2 - Automatic focusing device for scanning electron microscope - Google Patents

Description

The present invention relates to an automatic focusing device in a scanning electron microscope.

[Conventional technology]

A conventional automatic focusing device in a scanning electron microscope will be described with reference to FIGS.

In FIG. 4, an electron beam 2 emitted from an electron gun 1 is scanned by a scanning coil 3 in the X and Y directions, and
Irradiate the surface of the sample 6 with the objective lens 5 so as to focus it,
At this time, the secondary electron 7 generated from the sample surface is detected by the detector 8. When scanning with the electron beam 2 as shown in FIG.
A change in the detection signal occurs at the sample edge 20, but this change becomes maximum when the objective lens is in focus and the electron beam spot diameter is the minimum. The line spot diameter increases and the signal change decreases. Therefore, the detection signal is passed through the filter 9 to extract a high-frequency component, the extracted signal is integrated over the scanning range, the computer 11 sequentially changes the objective lens current within a predetermined width, and this integration is repeated to obtain each objective lens current. Extract the integrated value. Then, when the objective lens current is plotted on the horizontal axis and the signal integration value is plotted on the vertical axis, the characteristic 21 in FIG. 6 is obtained,
The objective lens current F0 that maximizes the signal integration value is obtained as the focus position.

[Problems to be solved by the invention]

However, in general, the shape of the sample has a large directivity, and the objective lens current that maximizes the signal integration value does not always reach the true focus position when the electron beam has astigmatism.
For example, a sample edge with directionality as shown in FIG.
When there is 30 and the scanning direction is the direction of the arrow in the figure, the electron beam spot 31 has an elliptical shape due to astigmatism and its major axis is parallel to the sample edge (FIG. 7 (a)). When the spot 32 is circular at the in-focus position (Fig. 7 (b)), the spot 33 is elliptical due to astigmatism and its major axis is orthogonal to the sample edge (Fig. 7). (C)), the signal integral value becomes maximum in the case of FIG. 7 (a), and the signal integral value becomes smaller in the order of FIG. 7 (b) and FIG. The signal integral value with respect to is the characteristic 22 in FIG. 6, and the objective lens current F1 is at the focal point. As described above, in the conventional automatic focusing method, the correct in-focus position can be obtained only when the astigmatism is correctly corrected, and the correct in-focus position cannot be obtained when the astigmatism is not correctly corrected. . On the contrary, astigmatism correction cannot be completely performed unless it is correctly focused.

The present invention is for solving the above-mentioned problems, and is a scanning electron microscope capable of obtaining a correct in-focus position regardless of an astigmatism correction amount before operation and even for a sample having a directional shape. It is an object of the present invention to provide an automatic focusing device.

[Means for Solving the Problems]

Therefore, the automatic focusing device for a scanning electron microscope according to the present invention includes an electron beam scanning unit that scans an electron beam, an objective lens driving unit, and a detection unit that detects an electron current obtained from a sample irradiated with the scanning electron beam. Means, integrating means for integrating the high frequency component of the signal detected by the detecting means, and control means for controlling the objective lens driving means to change the objective lens current according to the integration result in the integrating means, In an automatic focusing device for a scanning electron microscope, wherein the control means changes the objective lens current to obtain the objective lens current that maximizes the integration result, and the astigmatism is synchronized with the electron beam scanning by the scanning means. An astigmatism correction coil current scanning means for changing the aberration correction coil current is provided, and the astigmatism correction coil current is supplied during electron beam scanning for focusing. Characterized in that so as to vary.

[Action]

The present invention scans the astigmatism correction coil current during the electron beam scanning on the sample surface to positively give the effects of astigmatism of all intensities equal to each other, so that the effects of the astigmatism and the directional property of the sample are obtained. If the position where the signal integration value is maximized is obtained by eliminating the above, the correct focus position can be obtained regardless of the astigmatism correction amount before the operation and even for a sample having a directional shape.

〔Example〕

 Hereinafter, embodiments will be described with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining a focusing method of the present invention, and FIG. 3 is a diagram showing a relation between an objective lens current and a detection signal integral value. .
In the figure, 50 is an electron gun, 51 is a scanning coil, and 51a is horizontal (H).
Directional scanning coil, 51b is vertical (V) directional scanning coil, 52
Is an astigmatism correction coil, 52a is an X direction correction coil, 52b is a Y direction correction coil, 53 is an objective lens, 55 is an electron beam, 56 is a sample, 57 is a detector, 59 is a high-frequency component extraction filter, and 60 is Integrator, 61 A / D converter, 62 sampling value storage circuit, 6
3 is an arithmetic processing control device, 65 is an objective lens power supply, 66 is a scanning circuit, 67 is an astigmatism correction coil power supply, and 70 is a scan area.

The scanning coil 51 is driven by the scanning circuit 66 to scan the electron beam 55 in the H direction and the V direction over the scan region 70 as shown in FIG. The astigmatism correction coil 52 is driven by an astigmatism correction coil power supply 67 controlled by a scanning circuit 66, and scans while continuously changing the amount of astigmatism correction over a sufficiently wide range in synchronization with electron beam scanning. To do. The objective lens 53 is driven by an objective lens power source 65 controlled by the arithmetic processing control device 63, and the objective lens current is sequentially changed at a predetermined step every time the scanning of the scan area is completed. The detector 57 detects the secondary electrons from the sample 56, and the high frequency component of the detection signal is extracted by the filter 59. Integrator 60, A /
The timing of the D converter 61 is controlled by the scanning circuit 66, and the integrated value of the signal is A / D converted and stored in the sampling value storage circuit 62 each time the scanning of the scan region 70 is completed.

In such a configuration, the electron beam 55 from the electron gun 50 is scanned by the scanning coil 51 to irradiate the sample surface, and in synchronization with the electron beam scanning, the X astigmatism correction coil 52a, as shown in FIG.
The current supplied to the Y astigmatism correction coil 52b is changed over a sufficiently wide range to create all combinations of astigmatism. In this way, the secondary electrons generated from the sample 56 are detected by the detector 57, the high frequency component is extracted by the filter 59, the signal is integrated by the integrator 60 over the scan region 70, and the A / D converter 61.
A / D conversion is performed with, and the sampling value storage circuit samples and stores. In this case, since the signal is originally small when the focus is very far, the integrated value is also small,
Also, if there are various edge directions on the sample, they will be averaged and will not affect the integrated value. Further, even if the edge has directionality, all astigmatisms are created in one integration, so they are averaged, and as a result, the effect of astigmatism disappears, and only the effect of focus becomes apparent.
Therefore, when the signal integration value is obtained with the astigmatism correction amount fixed, the signal integration value becomes, for example, a curve 72 in FIG. 3 due to the influence of astigmatism and edge directionality. , By scanning the astigmatism correction coil current, the influence of astigmatism on the directionality of the edge disappears and the curve becomes as shown by the curve 71 in FIG. 3, and the maximum focus position F0 is calculated to determine the in-focus position. You can Therefore, the arithmetic and control unit 63 stores the integrated value when the objective lens current is sequentially changed, and the objective lens current that maximizes the integrated value is obtained to determine the influence of astigmatism and the influence of the directionality of the edge. Without it, it becomes possible to obtain the in-focus position.

〔The invention's effect〕

As described above, according to the present invention, it is possible to eliminate the influence of astigmatism and the directivity of the sample by positively making astigmatism of any strength and performing electron beam scanning, and The correct in-focus position can be easily obtained regardless of the astigmatism correction amount and even for a sample having a directional shape.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining a focusing method of the present invention, FIG. 3 is a diagram showing a relation between an objective lens current and a detection signal integral value, FIG. FIG. 4 is a diagram for explaining a configuration of a conventional automatic focusing system, FIG. 5 is a diagram for explaining scanning of an electron beam, FIG. 6 is a diagram showing a relationship between an objective lens current and a signal integral value, FIG. 7 is a diagram for explaining the influence of the sample shape and astigmatism. 51 …… Scanning coil, 52 …… Astigmatism correction coil, 53 ……
Objective lens, 55 ... electron beam, 56 ... sample, 57 ... detector, 59 ... high-frequency component extraction filter, 60 ... integrator, 62 ...
… Sampling value storage circuit, 63 …… Arithmetic processing control device,
65 ... Objective lens power supply, 66 ... Scanning circuit, 67 ... Astigmatism correction coil power supply, 70 ... Scan area.

Claims (1)

[Claims] 1. An electron beam scanning means for scanning an electron beam, an objective lens driving means, a detecting means for detecting an electron current obtained from a sample irradiated with a scanning electron beam, and a signal detected by the detecting means. An integrating means for integrating the high frequency component, and a control means for controlling the objective lens driving means to change the objective lens current according to the integration result in the integrating means are provided, and the objective lens current is changed by the controlling means, In an automatic focusing device for a scanning electron microscope, which is configured to obtain an objective lens current that maximizes an integration result, further, astigmatism correction for changing an astigmatism correction coil current in synchronization with electron beam scanning by the scanning means. A scanning electron beam scanning device comprising a coil current scanning means for changing the astigmatism correction coil current during scanning of an electron beam for focusing. Microscope automatic focusing device.