JPH0255900B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in automatic focusing devices used in scanning electron microscopes and similar devices.

In a scanning electron microscope, as shown in FIG. 1, an electron beam 2 generated from an electron gun 1 is passed through a focusing lens 3,
4, the electron beam 2 is focused to form a minute cross-sectional diameter on the surface of the sample 5, and at the same time, the electron beam 2 is scanned on the sample surface by a scanning signal supplied to the deflection coils 6X, 6Y. A sample scanning image is displayed by supplying a video signal from the sample detected by a detector 8 and amplified by an amplifier 9 as a brightness modulation signal to a cathode ray tube 7 in synchronization with the scanning. The image magnification of this sample scanning image is determined by the deflection coil 6.
X, 6Y and cathode ray tube deflection coil 10X, 10
The output terminal of the scanning power supply 11 that supplies the horizontal scanning difference signal H and the vertical scanning signal V to Y, and the deflection coil 6X,
The switching is performed by changing the amplification degree of the variable amplifier 12 inserted between the 6Y and 6Y. That is, the image magnification is adjusted by changing the ratio of the electron beam scanning area on the sample surface to the area on the cathode ray tube surface similar to the area. Focusing of the scanning image displayed on the cathode ray tube screen is performed by the excitation power supply 13 of the focusing lens, especially the final stage focusing lens (objective lens) 4.
This is done by adjusting the output of the electron beam to make sure that the cross-sectional diameter of the electron beam on the sample surface is minimized, but since this adjustment is based on the naked eye observation of the sample scanned image, this adjustment has recently been Automatic focusing devices have been put into practical use to automate this process. In FIG. 1, the block indicated by 14 is an automatic focusing circuit for performing such automatic focusing, and it sequentially changes the output of the excitation power supply 13 based on the trigger signal from the scanning power supply 11, and also changes the output of the excitation power supply 13 based on the trigger signal from the scanning power supply 11. An appropriate output signal of the excitation power source 13 is determined based on the result of monitoring the output of the excitation power source 13.

Figures 2 and 3 are for explaining the principle of the automatic focusing circuit 14, and Figure 2a shows a structure 15 with a certain width on the sample surface viewed from the optical axis direction.
The figure shows horizontal scanning of an electron beam 17 having the cross-sectional shape shown in the figure in the direction of an arrow 16 that crosses the structure, and the signal waveform detected from the sample by such electron beam scanning is shown in Figure 2b.
In FIG. 3a, the same sample scanning as in FIG. 2a is performed using an electron beam 18 having a different cross-sectional diameter, and FIG. 3b shows the signal waveform obtained by this scanning. From these figures, it can be seen that the smaller the cross section of the electron beam scanning the sample, the higher the height of the signal waveform, and the sharper the peak waveform becomes. Conventional automatic focusing devices utilize this type of development, and extract only the high frequency components of signals detected by electron beam sample scanning using differentiating circuits and filter circuits, and calculate the peak value or peak value of the high frequency components. In most cases, the integrated value is regarded as a signal corresponding to the cross-sectional diameter of the electron beam, and the excitation of the objective lens is set so that these peak values or integrated values are maximized.

Using the conventional equipment described above does not necessarily mean that it is possible to completely automate the focusing of a scanning electron microscope. The reliability of automatic focusing devices has been quite low because automatic focusing often does not function due to reasons such as inappropriate scanning speed or observation magnification.

The purpose of the present invention is to improve the reliability of such conventional devices.The purpose of the present invention is to two-dimensionally scan the sample surface with an electron beam narrowly focused by a focusing lens. A device that detects a signal generated from a sample using a detector and a filter that cuts out low frequency components, and automatically adjusts the excitation of the focusing lens by regarding the detected signal as the cross-sectional diameter of the electron beam on the sample surface. The present invention is characterized in that means is provided for increasing/decreasing the cut frequency of the filter in conjunction with the operation for increasing/decreasing the scanning area on the sample surface by the electron beam.

FIG. 4 is a schematic diagram showing an apparatus according to an embodiment of the present invention, and the same reference numerals as in FIG. 1 represent the same components. The device shown in FIG. 4 is significantly different from the device shown in FIG.
The circuit 21 has almost the same function as the automatic focusing circuit 14 in FIG. The variable filter is linked to the operation of a variable amplifier 12 for controlling image magnification, and its characteristics are as shown in FIG. As can be seen from FIG. 5, as the image magnification decreases from x30000 to x3000 to x300, the frequency to be cut is shifted lower. Therefore, even if the image magnification at the time of starting the automatic focusing operation is different, the signal input to the automatic focusing circuit is always suitable for monitoring the cross-sectional diameter of the electron beam for the following reasons. become.

Generally, if the excitation (focus) of the objective lens is shifted by a certain amount from the correct focusing, the high frequency components contained in the detection signal from the sample will decrease, and the
The trends shown in the figure can be seen. In FIG. 6, the horizontal axis represents the excitation intensity of the objective lens, and the vertical axis represents the detection signal taken out through the filter circuit. The three curves shown in Figure 6 each have an image magnification of 30,000 times,
This also shows the cases of 3000x and 300x, and the rate at which the detection signal decreases is smaller for lower magnification images. Also, if you want to increase the change in the detection signal due to a slight change in excitation, you can lower the detection lower limit frequency of the filter, but if you lower the lower limit too much, the detection signal will be affected by noise more often. Become. The inventor confirmed this phenomenon, and
We assumed that preferable results could be obtained by increasing the lower limit of the high-frequency components to be extracted in accordance with the increase in image magnification, and we confirmed through experiments that this assumption was correct. Therefore, if the lower detection limit frequency of the filter is changed according to the image magnification, as in the example device shown in FIG. The reliability of the alignment device can be further improved.

In order to achieve accurate focusing, it is necessary that the astigmatism correction device normally built into scanning electron microscopes is properly adjusted to correct astigmatism in the focusing lens system. ,
In order to achieve precise automatic focusing, it is also necessary to automate the adjustment of the astigmatism correction device. The peak value or integrated value of the high frequency component of the signal detected from the sample is used as information to confirm whether or not this astigmatism correction device is correctly adjusted. It is extremely effective to increase and decrease in conjunction with variable operation.

As described above, according to the present invention, it is possible to improve the reliability of an automatic focusing device incorporated in an electron beam device, and it can greatly contribute to improving the operability of the electron beam device.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the configuration of a general automatic focusing device, Figs. 2 and 3 are schematic diagrams for explaining the principle of the device in Fig. 1, and Fig. 4 is an embodiment of the present invention. Schematic diagrams showing the apparatus, Figures 5 and 6 are
3 is a schematic diagram for explaining the operation of the device shown in the figure. 1: Electron gun, 2: Electron beam, 3, 4: Focusing lens, 5: Sample, 7: Braun tube, 8: Detector, 1
1: Scanning power supply, 12: Variable amplifier, 1: Excitation power supply, 14: Automatic focusing circuit, 17, 18: Electron beam, 19: Automatic focusing circuit, 20: Variable filter.

Claims (1)

[Claims] 1 The sample surface is scanned two-dimensionally with an electron beam narrowly focused by a focusing lens, and the signal generated from the sample due to the scanning is detected by a detector and a filter that cuts out low frequency components. , in a device that automatically adjusts the excitation of the focusing lens by regarding the detected signal as the cross-sectional diameter of the electron beam on the sample surface; An automatic focusing device for a scanning electron microscope, characterized in that it is provided with means for changing the cut frequency of a filter.