JPH08138605A - Focusing method and astigmatism correcting method in electron beam device, and electron beam device - Google Patents

Abstract

PURPOSE: To provide focusing and astigmatism correcting methods in such an electron beam device that highly accurately performs focusing and correcting astigmatism in a short time even in a sample having a characteristic region only in a specified part of the sample, and to provide such an electron beam device. CONSTITUTION: Thinning raster scanning is performed in a prescribed area of a sample 4 before focusing and astignatism correction action. A signal obtained from a detector 6 based on each raster scanning is supplied to an integrator 11 so as to perform integration processing according to each scanning line. The integration results according to each line are stored in a memory 13. A control circuit 14 compares the integration results according to each scanning line, which are stored in the memory 13 and a scanning line on which the maximum strength is obtained is detected. The control circuit 14 specifies a prescribed width of a scanning region including this scanning line based on the detected scanning line and controls a deflection control circuit 17 so as to perform focusing and astigmatism.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing method and astigmatism correction in an electron beam apparatus such as a scanning electron microscope which is optimal for automatically focusing an electron beam and correcting astigmatism. The present invention relates to a method and an electron beam device.

[0002]

2. Description of the Related Art A scanning electron microscope has an automatic focusing function. In this focusing, the excitation of the focusing lens is changed stepwise, and a predetermined region of the sample is scanned with the electron beam in each excitation state, that is, in each focusing state of the electron beam. Electrons and backscattered electrons are detected, and detection signals are integrated for each focusing state. Then, the integrated values of the detection signals in each focusing state are compared, the focusing state when the maximum value is obtained is determined to be the focus position, and the excitation of the focusing lens is set in that state.

[0003]

By the way, in focusing, if unevenness is present over the entire scanning region of the sample, highly accurate focusing can be performed. However, in the case of a sample in which a characteristic part exists in a part of the scanning region of the electron beam and the other part is a smooth plane like the IC pattern, the detection signal used for focusing is The difference in the integrated value may be small depending on each focusing state of the electron beam, and the focusing accuracy may be lowered.

In many cases, the scanning shape of the electron beam at the time of focusing in the conventional scanning electron microscope is an all-pixel type scanning in which a predetermined sample area is raster-scanned. FIG.
Shows the situation of such scanning, and S shows the scanning region of the electron beam of the sample. The electron beam is raster-scanned on the scanning region S as in D1 to Dn. Here, when the characteristic portion P such as the unevenness exists only in a part of the sample region S, the difference between the integrated values of the detection signals used for focusing becomes small depending on each focusing state of the electron beam, and the focusing is performed. As described above, the accuracy of may decrease. Further, in such a case, the raster scanning other than the scanning line Dk that crosses the characteristic portion P does not substantially contribute to the focusing, so that the time required for the scanning is wasted. Such a problem also occurs when the astigmatism of the electron beam is corrected.

The present invention has been made in view of the above circumstances, and an object thereof is to perform focusing with high accuracy in a short time even in a sample in which a characteristic region exists only in a specific portion of the sample. It is to realize a focusing and astigmatism correction method in an electron beam apparatus and an electron beam apparatus capable of correcting astigmatism and astigmatism.

[0006]

A focusing and astigmatism correction method in an electron beam apparatus and an electron beam apparatus according to the present invention scan an electron beam in a predetermined scan unit in advance in a first sample region. , A scanning unit having the maximum signal intensity is selected from the signal intensities of the respective scanning units, and focusing and astigmatism correction operations are executed in the second sample region based on the selected scanning unit. Is characterized by.

[0007]

In the present invention, prior to the focusing and astigmatism correction operations, the electron beam is scanned in the predetermined sample unit in advance in the first sample region, and the maximum signal intensity is obtained from the signal intensity of each scan unit. The scanning unit for which the signal intensity is obtained is selected, and the focusing and astigmatism correction operation are executed in the second sample area based on the selected scanning unit.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 2 shows a scanning electron microscope which is an embodiment of the present invention, and 1 is an electron gun. The electron beam EB generated from the electron gun 1 is focused on the focusing lens 2 and the objective lens 3.
Then, it is finely 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 supplied to the high-pass filter 8 and the cathode ray tube 9.

The signal passed through the high pass filter 8 is supplied to the integrator 11 via the absolute value circuit 10. Integrator 1
The integration result of 1 is stored in the memory 13 via the AD converter 12.
Is supplied to. The signal stored in the memory 13 is appropriately supplied to the control circuit 14. Reference numeral 15 is an operation panel, and the operation panel 15 sends an instruction signal to the control circuit 14. Control circuit 14
Controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 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 deflection control circuit 17 based on an instruction signal from the operation panel 15, and the deflection control circuit 17 sends a predetermined scanning signal to the deflection coil 5. It is supplied, and an arbitrary two-dimensional area on the sample 4 is raster-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 9 synchronized with the scanning signal to the deflection coil 5 through the amplifier 7, and the secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube 9.

Next, a case where a normal electron beam focusing operation is performed will be described. When the operation panel 15 is operated to instruct the focusing mode, the control circuit 14 controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 for the deflection coil 5. As a result, the driving power supply 16 supplies the objective lens 3 with a stepwise exciting current, and the deflection control circuit 17 sends a scanning signal for scanning a predetermined region of the sample each time the stepwise exciting current changes. Deflection coil 5
Supply to.

The secondary electron signal detected by the detector 6 in the focus state by each step-like exciting current is amplified by the amplifier 7, the direct-current component is removed by the high-pass filter 8, and then the absolute value circuit. It is converted by 10 into a positive signal. The output of the absolute value circuit 10 is supplied to the integrator 11, and the signal based on one scanning of the electron beam for each excitation step of the objective lens 3 is integrated. The integrated value of the integrator 11 is converted into a digital signal by the AD converter 12, and then supplied to the memory 13.

The memory 13 stores the integrated value of the integrator 11 for each excitation step of the objective lens 3.
FIG. 3 shows the change in the stored integral value at this time,
The vertical axis represents the integrated value, and the horizontal axis represents the excitation intensity of the objective lens 3.
The control circuit 14 smooths the change curve of the integrated value stored in the memory 13 and then performs fitting with a curve made up of a quadratic function to excite the objective lens 3 at the peak of the change in the integrated value of the signal amount. Detect intensity. Control circuit 14
Controls the driving power supply 16 to set the objective lens 3 to the excitation intensity at the peak, and the focusing operation is performed in this way.

Prior to the above-mentioned focusing operation, the selection processing of the scanning area of the electron beam is executed. First, the deflection control circuit 17 is controlled by the control circuit 14 and thinning scanning is performed in a predetermined region of the sample 4. FIG.
Shows the state of this thinning-out scanning. In the scanning region S of the sample, the electron beam is thinned-out scanning at intervals of raster scanning like D1 to Dm. The signal obtained from the detector 6 based on each raster scan is supplied to the integrator 11 to be integrated for each scanning line. The integration result for each line is stored in the memory 13.

The control circuit 14 compares the integration results for each scanning line stored in the memory 13 and detects the scanning line Dh where the maximum intensity is obtained. Based on the detected scanning line Dh, the control circuit 14 designates a scanning region of a predetermined width including this scanning line and controls the deflection control circuit 17. FIG. 5 shows the selected scanning region Ss having a predetermined width, and the above-described focusing operation of the electron beam is performed based on the scanning of the electron beam in the scanning region Ss. That is, the excitation intensity of the objective lens 3 is changed stepwise each time scanning is performed in the scanning region Ss, and the in-focus position is obtained from the integration result of the signals detected based on the scanning of the electron beam for each excitation intensity. To be In this focusing operation, since the characteristic portion P of the sample is included in the scanning region Ss of the electron beam and the proportion of the smooth sample portion is small, the integrated value of the detection signal used for focusing is Further, the difference becomes large depending on each focus state of the electron beam, and the focusing accuracy can be improved. Further, since the electron beam is not scanned on the smooth portion of the sample that does not substantially contribute to focusing, focusing can be performed in a relatively short time.

FIG. 6 shows another embodiment of the present invention.
The same or similar parts as those of the first embodiment shown in FIG. 2 are designated by the same reference numerals. In this embodiment, astigmatism correction is performed, and an astigmatism correction coil 18 is provided. An astigmatism correction signal is supplied to the astigmatism correction coil 18 from a power supply 19. The power supply 19 is controlled by the control circuit 14. In such a configuration, when the operation panel 15 is operated to instruct the astigmatism correction mode, the control circuit 14 causes the drive power source 19 for the astigmatism correction coil 18 and the deflection control circuit 1 for the deflection coil 5.
7 and control. As a result, the driving power supply 19 supplies the astigmatism correction signal that changes stepwise to the astigmatism correction coil 18, and the deflection control circuit 17 scans a predetermined area of the sample each time the stepwise astigmatism signal changes. And a scanning signal for supplying the scanning signal to the deflection coil 5.

Detector 6 in each step-shaped astigmatism state
The secondary electron signal detected by is amplified by the amplifier 7, the direct current component is removed by the high-pass filter 8, and then converted into a positive signal by the absolute value circuit 10. The output of the absolute value circuit 10 is supplied to the integrator 11, and the signal based on one electron beam scan for each change step of the astigmatism correction signal to the astigmatism correction coil 18 is integrated. The integrated value of the integrator 11 is converted into a digital signal by the AD converter 12, and then supplied to the memory 13.

The memory 13 stores the integrated value of the integrator 11 for each signal change step of the astigmatism correction coil 18. The control circuit 14 detects the astigmatism signal when the maximum intensity is obtained from the signals stored in the memory 13, and controls the power supply 19 to supply the astigmatism signal at that time to the astigmatism correction coil 18. To do. In this way, the astigmatism correction operation is performed. Prior to such an astigmatism correction operation, also in this embodiment, the scanning area is selected as in the embodiment of FIG. That is, the thinning scanning of the electron beam is first performed in a predetermined scanning region, the scanning line having the maximum signal intensity is selected based on the thinning scanning, and the astigmatism correction is performed in the vicinity of the selected scanning line. The electron beam is scanned.

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 raster scanning is thinned out in order to select the scanning area, the predetermined scanning area is virtually divided into square unit scanning blocks for the selection of the scanning area, and the detection for each unit scanning block is performed. The signals may be compared, and the focusing and astigmatism correction operations may be performed in the unit scanning block in which the maximum signal strength is obtained. Furthermore, although the excitation of the objective lens was changed during the focusing operation, an auxiliary lens of the objective lens may be provided and the excitation of the auxiliary lens may be changed. Furthermore, when focusing, the detection signals are integrated for each scanning and the integrated signals are compared to obtain the optimum focus position. However, the maximum amplitude value (peak-to-peak value) of the detection signal is detected. It may be done.

[0020]

As described above, according to the present invention, the electron beam is scanned in a predetermined scanning unit in advance in the first sample region prior to the focusing and the astigmatism correction operation, and each scanning unit is scanned. The scanning unit that gives the maximum signal intensity is selected from the signal intensities for each
Focusing and astigmatism correction operations were performed in the sample area of. As a result, when performing focusing or astigmatism correction operation, the scanning area of the electron beam at that time surely includes the characteristic parts of the sample such as unevenness, and the ratio of the smooth sample part becomes small. Therefore, the integrated value of the detection signal used for focusing and astigmatism correction has a large difference depending on each focus state of the electron beam, and the accuracy of focusing and astigmatism correction can be improved. Further, since the electron beam is not scanned on the smooth portion of the sample that does not substantially contribute to focusing and astigmatism correction, focusing can be performed in a relatively short time.

[Brief description of drawings]

FIG. 1 is a diagram showing how an electron beam is scanned for focusing.

FIG. 2 is a diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 3 is a diagram showing a relationship between an excitation intensity of an objective lens and an integrated value.

FIG. 4 is a diagram showing a state of thinning scanning of an electron beam.

FIG. 5 is a diagram showing scanning of an electron beam in a selected sample region.

FIG. 6 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

 1 Electron Gun 2 Focusing Lens 3 Objective Lens 4 Sample 5 Deflection Coil 6 Detector 7 Amplifier 9 Cathode Ray Tube 12 AD Converter 14 Control Circuit 15 Operation Panel 16 Drive Power Supply 17 Drive Power Supply 18 Astigmatism Correction Coil 19 Power Supply

Claims (3)

[Claims] 1. The intensity of a focusing lens for focusing an electron beam on a sample is changed stepwise, and a predetermined region on the sample is scanned by the electron beam each time the intensity is changed. In the focusing method in the electron beam apparatus in which the intensity of the optimum focusing lens is detected on the basis of the signal and the focusing lens intensity is set to that intensity, in advance for each predetermined scanning unit in the first sample region. Scanning with an electron beam is performed, a scanning unit having the maximum signal intensity is selected from the signal intensities for each scanning unit, and the focusing operation is performed in the second sample region based on the selected scanning unit. A focusing method in an electron beam apparatus, characterized in that 2. A signal to a correction means for correcting astigmatism of an electron beam is changed stepwise, and a predetermined region on a sample is scanned with the electron beam each time the signal is changed. In the astigmatism correction method in the electron beam device, which detects the optimum signal strength to the astigmatism correction means based on the signal and sets the strength of the correction signal to the astigmatism correction means in advance, The electron beam is scanned in a predetermined scan unit in the first sample region, the scan unit that gives the maximum signal intensity is selected from the signal intensities of the respective scan units, and the second scan unit based on the selected scan unit is selected. The astigmatism correction method in the electron beam apparatus, wherein the astigmatism correction operation is executed in the sample region. 3. A focusing lens for focusing the electron beam on the sample, a means for correcting the astigmatism of the electron beam, and a predetermined region on the sample composed of a plurality of unit scanning regions for scanning with the electron beam. Comparing the scanning means, the detector that detects the signal obtained by irradiating the sample with the electron beam, the integrating means that integrates the signals from the detector, and the integrating results of the integrating means for each unit scanning area of the electron beam And a means for selecting a scanning area of the sample for focusing or astigmatism correction of the electron beam based on the unit scanning area where the maximum integration result is obtained.