JPH03219543A - Automatic focusing system of scanning type electron microscope - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic focusing system in a scanning electron microscope.

[Conventional technology]

A conventional automatic focusing system in a scanning electron microscope will be explained with reference to FIGS. 4 to 6.

In FIG. 4, an electron beam 2 emitted from an electron gun 1 is scanned in the X direction and the Y direction by a scanning coil 3, and
Irradiate the surface of the sample 6 with the objective lens 5 so as to focus it on the surface of the sample 6,
At this time, secondary electrons 7 generated from the sample surface are detected by a 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 is maximum when the objective lens is focused and the electron beam spot diameter is minimum, resulting in underfocus,
As the electron beam becomes out of focus due to overfocus, the diameter of the electron beam spot on the sample surface increases, and the change in the signal decreases. Therefore, the detection signal is passed through the filter 9 to extract high frequency components, the extracted signal is integrated over the scanning range, and the computer 11 sequentially changes the objective lens current in a predetermined width and repeats this integration. Extract the integral value. If the horizontal axis is the objective lens current and the vertical axis is the signal integral value, it becomes characteristic 21 in FIG. Problems to be Solved] However, the shape of the sample is generally thick in directionality (and in a state where the electron beam has astigmatism, the objective lens current where the signal integral value is maximum does not necessarily correspond to the true focal point position.

For example, when there is a directional sample edge 30 as shown in FIG. 7 and the scanning direction is in the direction of the arrow in the figure, the electron beam spot 31 has an elliptical shape due to astigmatism, and its long axis is parallel to the specimen edge (Fig. 7(a)), and when the spot 32 is circular at the focused position (Fig. 7(b)), the spot 33 is elliptical due to astigmatism. When the long axis is perpendicular to the sample edge (Figure 7 (C))
Assuming that, the signal integral value is maximum in the case of Fig. 7 (a), and the signal integral value decreases in the order of Fig. 7 (b) and Fig. 7 (Cal), and the signal integral value with respect to the objective lens current is Characteristic 22 in Fig. 6 is obtained, and the objective lens current F1 ends up at the in-focus position.In this way, with the conventional automatic focusing method, the correct in-focus point is reached only when astigmatism is correctly corrected. If the position is obtained and astigmatism is not correctly corrected, the correct in-focus position cannot be obtained. Conversely, astigmatism cannot be completely corrected unless the focus is correct.

The present invention is intended to solve the above problems, and is a scanning electron microscope that is capable of determining the correct focal point position regardless of the amount of astigmatism correction before operation and even for samples with directional shapes. The purpose is to provide an automatic focusing method.

[Means to solve the problem]

To avoid this, the automatic focusing method of the scanning electron microscope of the present invention irradiates the sample with a scanning electron beam and detects the electron current from the sample for each objective lens current when the objective lens current is changed. An astigmatism correction coil that changes the astigmatism correction coil current during electron beam scanning in the automatic focusing method of a scanning electron microscope that calculates the integral value and sets the focal point position to the objective lens current that gives the maximum integral value. It is characterized by providing a current scanning means.

[Effect]

The present invention can improve the effects of astigmatism and sample direction by scanning the astigmatism correction coil current to actively apply the effects of astigmatism of all strengths equally during electron beam scanning on the sample surface. By eliminating this and finding the position that maximizes the signal integral value, it is possible to find the correct in-focus position regardless of the amount of astigmatism correction before operation, and even for samples with directional shapes. can.

〔Example〕

Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the focusing method of the present invention, and FIG. 3 is a diagram showing the relationship between the detection signal integral value and the objective lens current. .

In the figure, 50 is an electron gun, 51 is a scanning coil, 51a is a horizontal (H) direction scanning coil, 51b is a vertical (V) direction scanning coil, 52 is an astigmatism correction coil, 52a is an X direction correction Koinobe 52b is a Y Direction correction coil, 53 objective lens, 55 electron beam, 56 sample, 57 detector, 59 high frequency component extraction filter, 60 integrator, 61 A/D converter, 62 sampling value storage circuit , 63 is an arithmetic processing control unit, 65 is an objective lens power supply, 66 is a scanning circuit, 67
70 is an astigmatism correction coil power supply and a scan area.

The scanning coil 51 is driven by a scanning circuit 66 to scan the electron beam 55 in the H direction and the V direction across the scanning area 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 by continuously changing the amount of astigmatism correction over a sufficiently wide range in synchronization with electron beam scanning. do. The objective lens 53 is an objective lens power source 6 controlled by an arithmetic processing control device 63.
5, and changes the objective lens current sequentially in a predetermined step every time scanning of the scan area is completed. Detector 5
7 detects secondary electrons from the sample 56, and a filter 59 extracts high-frequency components of the detection signal. Integrator 60, A/D
The timing of the converter 61 is controlled by a scanning circuit 66, and the integral value of the signal is A/D converted and stored in the sampling position storage circuit 62 every time scanning of the scan area 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 currents supplied to the X astigmatism correction coil 52aSY stigmatization coil 52b are varied over a sufficiently wide range as shown in FIG. to create all kinds of astigmatism combinations.

In this way, the secondary electrons generated from the 5th day of the sample are detected by the detector 57, the high frequency components are extracted by the filter 59, the signal is integrated by the integrator 60 over the scan area 70, and the A/D
The converter 61 performs A/D conversion, and the sampling value storage circuit samples and stores the data. In this case, if the focus is very far away, the signal is originally small, so the integral value is also small, and if there are various edge directions on the sample, they are averaged out and have no effect on the integral value.

Furthermore, even if the edge has directionality, all astigmatism is created in one integration, so it is averaged out, and as a result, the effect of astigmatism disappears, and only the effect of focus becomes apparent. Therefore, when the signal integral value is determined with the astigmatism correction amount fixed, the signal integral value will become, for example, like the curve 72 in Figure 3 due to the influence of the astigmatism and the directionality of the edge. By scanning the astigmatism correction coil current, the influence of astigmatism on the directionality of the edge disappears, resulting in a curve 71 in FIG. 3, and by finding its maximum position FO, the in-focus position can be found. I can do it. Therefore, by storing the integral values obtained when the objective lens current is sequentially changed using the arithmetic and control unit 63, and determining the objective lens current that maximizes the integral value, the influence of astigmatism and the influence of edge directionality can be reduced. It becomes possible to find the focal point position without any need for the lens.

〔Effect of the invention〕

As described above, according to the present invention, by actively creating astigmatism of any strength and performing electron beam scanning, it is possible to eliminate the effects of astigmatism and sample directionality, and Regardless of the amount of astigmatism correction, even if the sample has a directional shape, it is possible to easily find the correct focal point position.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the focusing method of the present invention, FIG. 3 is a diagram showing the relationship between the detection signal integral value and the objective lens current, and FIG. Figure 4 is a diagram for explaining the configuration of a conventional automatic focusing system, Figure 5 is a diagram for explaining electron beam scanning, Figure 6 is a diagram showing the relationship between objective lens current and 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... Scanning area. Applicant: JEOL Ltd.

Claims (1)

[Claims] (1) When the sample is irradiated with a scanning electron beam and the objective lens current is changed, the electron current from the sample for each objective lens current is detected and the integral value is determined, and the objective lens current that gives the maximum integral value is determined. A scanning electron microscope characterized in that, in an automatic focusing system of a scanning electron microscope, the scanning electron microscope is equipped with an astigmatism correction coil current scanning means for changing an astigmatism correction coil current during electron beam scanning. automatic focusing method.