JPH05275042A - Focusing method for scanning electron microscope - Google Patents

Abstract

PURPOSE:To realize a focusing method in a scanning electron microscope which can correctly focus on a remarked part of a sample in a short time. CONSTITUTION:While displaying a secondary electron image in an optional region of a sample 4 on a cathode ray tube 16, a luminescent line signal from a luminescent line generating circuit is supplied to an adder 9. As a result, a rectangular luminescent line L superimposed on a secondary electron image is displayed on a screen of a cathode-ray tube 16. A position of this rectangle can be optionally set up by operating a consol panel 17. An operator moves a rectangular luminescent line to a part desired to be focused while observing the secondary electron image. A control circuit 14 obtains an information on the rectangular luminescent line L so as to control driving power supply 19 based on this information in order to perform scanning of an electron beam only in a sample region surrounded by the rectangular luminescent line. Further, the control circuit 14 controls the driving power supply 18 so as to stepwise change the excitation strength of an object lens 3. In this state, focusing operation is performed.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In focusing of a scanning electron microscope or the like,
The excitation of the objective lens is changed stepwise, and in a stepped excitation state of the objective lens, a specific region of the sample is scanned with an electron beam, and secondary electrons or backscattered electrons from the sample are detected for each scanning. From this detected signal strength, when the maximum value of the signal strength is set as the focus point, the objective lens is fixed in the excited state at that time.

[0003]

In the focusing described above, signals from the entire specific scanning area of the sample are used, and the focusing is performed by the average signal of the specific scanning area. As a result, it is preferable that the specific region of the sample has unevenness on average, but if the degree of unevenness is partially different, it becomes impossible to accurately focus on some points of interest in the specific region. For example, even if there is a portion with a large amount of unevenness and a relatively smooth portion in the specific scanning area and you want to focus on a relatively smooth portion to observe an image, conventional focusing does not cause unevenness. The focus tends to be on the intense part.

The present invention has been made in view of the above circumstances, and an object of the present invention is to realize a focusing method in a scanning electron microscope capable of accurately focusing on a target portion of a sample in a short time. It is in.

[0005]

A focusing method in a scanning electron microscope according to the present invention includes an objective lens for focusing an electron beam on a sample and a scanning for scanning an irradiation position of the electron beam on the sample. Means and a detector for detecting the signal obtained by irradiating the sample with the electron beam,
In a scanning electron microscope equipped with means for changing the focusing state of the electron beam on the sample in a stepwise manner by changing the excitation intensity of the objective lens, a predetermined region of the sample is scanned with the electron beam in each focusing state, At that time, the focus detected by processing the signal detected by the detector to obtain an evaluation value signal indicating the degree of focusing, and focusing the electron beam according to the evaluation value for each focusing state of each electron beam. In the alignment method, a scan image of an arbitrary area on the sample is obtained,
A feature is that a desired area in the scan image is selected and focusing is performed based on the detection signal of the selected area portion.

[0006]

A focusing method in a scanning electron microscope according to the present invention obtains a scan image of an arbitrary region on a sample, selects a desired region in this scan image, and based on a detection signal of the selected region portion. Focus on.

[0007]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an example of a scanning electron microscope for carrying out the method of the present invention, 1 being an electron gun.
The electron beam EB generated from the electron gun 1 is focused by the focusing lens 2
And the objective lens 3 finely focuses the light on the sample 4. The electron beam EB is deflected by the deflection coil 5, and the irradiation position of the electron beam on the sample 4 is scanned. Secondary electrons generated by the irradiation of the sample 4 with the electron beam are detected by the secondary electron detector 6. The detection signal of the detector 6 is amplified by the amplifier 7 and then supplied to the high-pass filter 8 and the adder 9. The signal that has passed through the high-pass filter 8 is supplied to the peak detection circuit 13 via the AD converter 12. The peak value detected by the peak detection circuit 13 is supplied to the control circuit 14.

The adder 9 adds the signal from the amplifier 7 and the signal from the bright line signal generating circuit 15 and supplies the added signal to the cathode ray tube 16. Reference numeral 17 denotes an operation panel, and the operation panel 17 sends an instruction signal to the bright line signal generation circuit 15 and the control circuit 14. The control circuit 14 controls the drive power supply 18 for the objective lens 3 and the drive power supply 19 for the deflection coil 5. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the control circuit 14 controls the drive power supply 19 based on an instruction signal from the operation panel 17, and the drive power supply 19 supplies a predetermined scanning signal to the deflection coil 5. , An arbitrary region on the sample 4 is scanned by the electron beam EB. Secondary electrons generated by irradiating the sample 4 with the electron beam are detected by the detector 6. The detection signal is supplied to the cathode ray tube 16 synchronized with the scanning signal to the deflection coil 5 via the amplifier 7, and the cathode ray tube 16 displays a secondary electron image of an arbitrary region of the sample.

Next, a case where a normal electron beam focusing operation is performed will be described. When the operation panel 17 is operated and a focusing mode is instructed, the control circuit 14 causes the drive power supply 18 for the objective lens 3 and the drive power supply 19 for the deflection coil 5 to operate.
And control. As a result, the drive power source 18 is operated by the objective lens 3
To the driving power supply 1
Reference numeral 9 supplies a scanning signal to the deflection coil 5 for operating a predetermined region of the sample each time the stepwise excitation current changes. The secondary electron signal detected by the detector 6 in the focus state by the stepwise excitation current is amplified by the amplifier 7, and then the direct current component is removed by the high-pass filter 8. Further, after being converted into a digital signal by the AD converter 12, the peak detection circuit 1
3 is supplied.

The peak detection circuit 13 stores the peak value obtained at each excitation step of the objective lens 3. FIG. 2 shows the change in the stored peak value at this time, where the vertical axis is the peak value and the horizontal axis is the excitation intensity of the objective lens 3. The peak detection circuit 13 detects the excitation intensity of the objective lens 3 when the value of the stored change curve of the peak value becomes maximum, and the excitation intensity value is supplied to the control circuit 14. The control circuit 14 controls the driving power supply 18 to set the objective lens 3 to the excitation intensity at the peak, and the focusing operation is performed in this manner.

Next, the focusing operation according to the present invention will be described. First, a secondary electron image of an arbitrary area of the sample 4 is displayed on the cathode ray tube 16 and the bright line signal from the bright line signal generating circuit 15 is supplied to the adder 9. As a result, a rectangular bright line L is displayed on the screen of the cathode ray tube 16 so as to overlap the secondary electron image, as shown in FIG. The position of this rectangle is
It can be set arbitrarily by operating the operation panel 17. The operator moves the rectangular bright line L to a portion to be focused while observing the secondary electron image. In addition,
At this time, the position and size of the rectangle can be arbitrarily changed by the operation panel 17. After the setting operation of the rectangular bright line to the portion to be focused is completed, the operation panel 17 is operated to instruct the focusing mode. By this instruction, the control circuit 14
Obtains information on the rectangular bright line L, controls the drive power supply 19 based on this information, and scans the electron beam only in the sample region surrounded by the rectangular bright line L. Further, the control circuit 14
The drive power source 18 is controlled to change the excitation intensity of the objective lens 3 stepwise. In this state, the same focusing operation as described above is performed. Therefore, focusing is performed based on the information of the sample area surrounded by the rectangular bright line L, so that the focus point of the sample is accurately focused.

FIG. 4 shows the essential parts of another embodiment of the present invention. The same parts as those of the embodiment of FIG. 1 are designated by the same reference numerals. In this embodiment, a switch circuit 20 (may be a gate circuit) is arranged between the amplifier 7 for amplifying the detection signal of the secondary electron detector 6 and the high pass filter 8. The switch circuit 20 uses the signal from the bright line signal generation circuit 15 to scan the sample area surrounded by the bright line with the electron beam EB.
The secondary electron detection signal is supplied to the high-pass filter 8 only when scanning is performed. As a result, the focusing operation is performed only by the secondary electrons from the sample region surrounded by the bright line, and the observation of the target point of the sample can be performed in a focused state.

FIG. 5 shows the essential parts of another embodiment of the present invention. The same parts as those of the embodiment of FIG. 1 are designated by the same reference numerals. In this embodiment, the secondary electron detection signal amplified by the amplifier 7 is supplied to and stored in the memory 21.
During the focusing operation, only the signal in the area surrounded by the bright line is read from the memory 21 and supplied to the high-pass filter 8 by the read control circuit 22 to which the reference signal is supplied from the bright line signal generation circuit 15. As a result, the focusing operation is performed only by the secondary electrons from the sample region surrounded by the bright line, and the observation of the target point of the sample can be performed in a focused state. Further, in this embodiment, since signal processing for focusing can be performed based on the signal obtained by scanning the electron beam once for each excitation intensity of each objective lens, the electron beam to the sample is The number of irradiations can be reduced and the influence of sample charge-up can be avoided. As a modification of this embodiment, the whole process can be performed by masking the outside of the specific region and making it unchanged.

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 attention point is selected on the cathode ray tube by being surrounded by the bright line, the luminance of the attention point may be increased. Furthermore, a rectangular bright line or the like may be displayed on a plurality of cathode ray tubes, and a plurality of points of interest may be selected instead of a single point. In that case, a plurality of points of interest may be prioritized for focusing, or the entire points of interest may be focused as a whole. In this way, it is possible to perform automatic focusing according to the purpose by comparing and calculating the attention areas.

In the above-described embodiment, the peak value of the detection signal passed through the high pass filter is detected, and this detection value is used as the evaluation signal representing the degree of focusing, but the detection signal passed through the high pass filter is used. May be supplied to the absolute value circuit and the output signal of the absolute value circuit may be integrated to use the signal representing the integrated value as the evaluation signal, or the detection signal may be differentiated and the absolute value of the differentiated signal may be integrated. The evaluation signal may be used as an evaluation signal, or the detection signal may be frequency-analyzed to obtain the ratio of high-frequency components, and the signal representing this ratio may be used as the evaluation signal.

Further, in the above-mentioned embodiment, the exciting current of the objective lens is changed stepwise in order to generate various focusing states. However, an auxiliary objective lens is arranged in the vicinity of the objective lens, and the auxiliary objective lens is arranged. The exciting current of the lens may be changed stepwise.

[0018]

As described above, the focusing method in the scanning electron microscope based on the present invention obtains a scan image of an arbitrary region on a sample, selects a desired region in this scan image, and selects the selected region. Since the focusing is performed based on the detection signal of the area portion, the focusing portion of the sample can be accurately focused in a short time.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a scanning electron microscope for carrying out the method of the present invention.

FIG. 2 is a diagram showing a relationship between an excitation intensity of an objective lens and an integrated value.

FIG. 3 is a diagram showing an image on a cathode ray tube and a bright line L.

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

FIG. 5 is a diagram showing another example of a scanning electron microscope for carrying out the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electron gun 2 Focusing lens 3 Objective lens 4 Sample 5 Deflection coil 6 Detector 7 Amplifier 8 High pass filter 9 Adder 12 AD converter 13 Peak detection circuit 14 Control circuit 15 Bright line signal generation circuit 16 Cathode ray tube 17 Operation panel 18, 19 Drive power supply

Claims (1)

[Claims] 1. An objective lens for focusing an electron beam on a sample, a scanning means for scanning an irradiation position of the electron beam on the sample, and a signal obtained by irradiating the sample with the electron beam. In a scanning electron microscope equipped with a detector for changing the excitation intensity of the objective lens and a means for changing the focusing state of the electron beam on the sample in a stepwise manner, a predetermined region of the sample is electronized in each focusing state. Scanning with a beam, at that time, the signal detected by the detector is processed to obtain an evaluation value signal indicating the degree of focusing, and the electron beam is focused according to the evaluation value for each focusing state of each electron beam. In the focusing method that I did,
A focusing method in a scanning electron microscope, wherein a scanning image of an arbitrary region on a sample is obtained, a desired region in the scanning image is selected, and focusing is performed based on a detection signal of the selected region portion.