JP3114335B2 - Focusing method in scanning electron microscope - Google Patents

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 such as a scanning electron microscope,
The excitation of the objective lens is changed stepwise, each energized when
In the (focusing state) , a specific region of the sample is scanned with an electron beam, secondary electrons or reflected electrons from the sample are detected for each scan, and the maximum value of the signal intensity from the detected signal intensity is used as a focus point. The objective lens is fixed in the excitation state at that time.

[0003]

In the focusing described above, signals from the entire specific scanning area of the sample are used, and focusing is performed by an average signal of the specific scanning area. As a result, it is good if the specific region of the sample has irregularities on average, but if the degree of the irregularities is partially different, the focus of a part of the specific region is not accurately focused. For example, when there are a portion with a large unevenness and a relatively smooth portion in a specific scanning area, and it is desired to observe an image by focusing on a relatively smooth portion, the conventional focusing is more effective than the conventional focusing. Intense parts tend to be in focus.

The present invention has been made in view of the above circumstances, and an object thereof 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]

SUMMARY OF THE INVENTION A focusing method in a scanning electron microscope according to the present invention comprises an objective lens for focusing an electron beam on a sample and a scanning method for scanning an irradiation position of the electron beam on the sample. Means, and a detector for detecting a signal obtained by irradiating the sample with the electron beam,
The scanning electron microscope the focusing state of the electron beam on the sample and means for changing stepwise by changing the excitation strength of the objective lens, specimen at each focusing state
It was scanned with an electron beam, obtained by the scanning detector
Detection signal processing the reputation value signal representative of the degree of focus, in the focusing method to perform focusing of the electron beam in accordance with the evaluation value for each focusing states of the electron beams, each focusing condition of the in scanned image of any area on the sample
The detection signal of the related detector is stored in a memory, and a desired area in the scanned image is selected, and the detection in the selected area is performed.
A signal is read from the memory, and focusing is performed based on the read detection signal.

[0006]

In the scanning electron microscope according to the present invention, the focusing method firstly focuses on an arbitrary area on the sample in each focusing state.
Scan and store the obtained detector signal in the memory
You. Next, a desired area in this scanned image is selected, and the selection is performed.
Reading the area detection signal from the memory;
Focusing is performed based on the detected signal.

[0007]

Embodiments of the present invention will be described below 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, where 1 is an electron gun.
The electron beam EB generated from the electron gun 1 is
And the objective lens 3 is narrowly focused on the sample 4. Further, 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 stored in the memory 2.
1 and stored. Also increased by the amplifier 7
The widened detection signal of the detector 6 is also supplied to the adder 9.
You. After that, the reference signal is supplied from the bright line signal generation circuit 15.
The read control circuit 22 reads the memory 21
And the detection signal is read out and supplied to the high-pass filter 8.
Is done. Signal passing through the high pass filter 8 is supplied to a peak detection circuit 13. 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 generation circuit 15 and supplies the added signal to the cathode ray tube 16. Reference numeral 17 denotes an operation panel. 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 a drive power supply 18 for the objective lens 3 and a drive power supply 19 for the deflection coil 5. The operation of such a configuration is as follows.

When a normal secondary electron image is observed, the control circuit 14 controls a driving power supply 19 based on an instruction signal from the operation panel 17, and a predetermined scanning signal is supplied from the driving power supply 19 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 to give an instruction of the focusing mode, the control circuit 14 supplies a drive power supply 18 for the objective lens 3 and a drive power supply 19 for the deflection coil 5.
And control. As a result, the drive power supply 18
To the drive power supply 1
Numeral 9 supplies a scanning signal to the deflection coil 5 for operating a predetermined region of the sample every time the step-like excitation current changes. The secondary electron detection signal detected by the detector 6 in the focus state by each step-like excitation current is supplied to the memory 21 after being amplified by the amplifier 7.
Is stored. Then, from the bright line signal generation circuit 15
The read control circuit 22 supplied with the reference signal
As a result, the detection signal stored in the memory 21 is read, and
After being supplied to the bypass filter 8 and removing the DC component, it is supplied to the peak detection circuit 13.

The peak detection circuit 13 stores a peak value obtained for each excitation step of the objective lens 3. FIG. 2 shows a change in the stored peak value at this time, in which the vertical axis represents the peak value and the horizontal axis represents 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 the 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 thus the focusing operation is performed.

Next, the focusing operation according to the present invention will be described. First, a secondary electron image of an arbitrary region of the sample 4 is displayed on the cathode ray tube 16, and a bright line signal from the bright line signal generating circuit 15 is supplied to the adder 9. As a result, as shown in FIG. 3, a rectangular bright line L is displayed on the screen of the cathode ray tube 16 so as to overlap the secondary electron image. The location 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 operation of setting a rectangular bright line to a portion to be focused is completed, the operation panel 17 is operated to give an instruction of a focusing mode.

In response to this instruction, the bright line signal generation circuit 15
From the read control circuit 22 to which the reference signal is supplied.
Therefore, only the detection signal in the area surrounded by the bright line L is stored in the memory
21 and is directly supplied to the high-pass filter 8.
After the stream is removed, it is supplied to the peak detection circuit 13.
You. Then, the control circuit 14 controls the driving power supply 18
The objective lens 3 is set to the excitation intensity at the peak. Ma
The control circuit 14 obtains information on the rectangular bright line L, and
The driving power supply 19 is controlled based on the
The scanning of the electron beam is performed only on the sampled area. Follow
Focusing is performed on the information of the sample area surrounded by the rectangular bright line L.
Focus on the sample's point of interest
Will fit.

[0014] In an embodiment of this, based on the detection signal obtained by performing a single scan of the electron beam for each excitation intensity of each objective lens (data in memory 21), the desired
Since the signal processing for focusing on the region can be performed as many times as possible, the number of times of irradiation of the sample with the electron beam can be reduced, and the influence of charge-up of the sample can be avoided. As a modification of this embodiment, the whole process can be performed as data that is not changed by masking the outside of the specific area.

Although the embodiment of the present invention has been described in detail, the present invention is not limited to this embodiment. For example, although secondary electrons are detected, reflected electrons may be detected. In addition, the point of interest is selected so as to be surrounded by a bright line on the cathode ray tube, but the luminance of the point of interest may be increased. Further, a configuration may be adopted in which a plurality of rectangular bright lines or the like can be displayed on a plurality of cathode ray tubes, and a plurality of points of interest can be selected instead of a single point. In this case, a plurality of attention points may be prioritized for focusing, or the focus of the entire attention points may be adjusted as a whole. As described above, by comparing and calculating the attention areas, automatic focusing according to the purpose can be performed.

In the above-described embodiment, the peak value of the detection signal that has passed through the high-pass filter is detected, and this detection value is used as the evaluation signal indicating the degree of focus. May be supplied to an absolute value circuit, an output signal of the absolute value circuit may be integrated, and a signal representing the integrated value may be used as an evaluation signal. Alternatively, the detection signal is differentiated, and the absolute value of the differentiated signal is integrated. Alternatively, the frequency of the detection signal may be analyzed to determine the ratio of the high frequency component, and a signal representing this ratio may be used as the evaluation signal.

In the above-described embodiment, the exciting current of the objective lens is changed stepwise in order to cause various focusing states. However, an auxiliary objective lens is arranged near the objective lens, and this 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 according to the present invention is performed in each focusing state.
Detection signal of detector for scanning image of arbitrary area on sample
Is stored in a memory, and a desired area in this scanned image is
Select and read the detection signal in the selected area from memory
And, since to perform focusing based on the detection signal thus read out, simply can not only perform accurate focusing on the target portion of a short time the sample, the objective
Performs one electron beam scan for each lens excitation intensity
Of the focusing based on the detection signal
Signal processing for the
The number of beam irradiations can be reduced,
The effect of page up can be avoided.

[Brief description of the drawings]

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

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

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

[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 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 21 Memory 22 Read control circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-54-48477 (JP, A) JP-A-1-264152 (JP, A) JP-A-59-165658 (JP, U) (58) Survey Field (Int.Cl. ⁷ , DB name) H01J 37/21 H01J 37/22

Claims (1)

(57) [Claims] An objective lens for focusing an electron beam on a sample, a scanning unit for scanning an irradiation position of the electron beam on the sample, and detecting a signal obtained by irradiating the sample with the electron beam. a detector for, in a scanning electron microscope equipped with a means for changing the focusing state of the electron beam on the sample stepwise by changing the excitation strength of the objective lens, a specimen with an electron beam at each converging state It scans, processes the detection signal of the detector obtained by this scanning, obtains an evaluation value signal indicating the degree of the focal point, and focuses the electron beam according to the evaluation value for each focusing state of each electron beam. In the focusing method, detection of a scanning image of an arbitrary area on the sample in each focusing state is performed.
The detection signal of the detector is stored in a memory, a desired area in this scan image is selected, and the detection signal in the selected area is described above.
A focusing method in a scanning electron microscope configured to read from a memory and perform focusing based on the read detection signal.