JP3114416B2 - Focusing method in charged particle beam device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing method for a charged particle beam apparatus such as a scanning electron microscope capable of automatically focusing a charged particle beam.

[0002]

2. Description of the Related Art A scanning electron microscope is provided with an automatic focusing function. In this focusing, the excitation of the focusing lens is changed stepwise, and a predetermined area 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 reflected electrons are detected, and a detection signal is integrated for each focusing state. Then, the integrated value of the detection signal in each focusing state is compared, the focusing state when the maximum value is obtained is determined as the focal point position, and the excitation of the focusing lens is set to that state.

[0003]

In the above-described focusing, if there is unevenness over the entire scanning area of the sample, highly accurate focusing can be performed. However, in the case of a sample, such as an IC pattern, in which a characteristic portion exists linearly in a part of the scanning region of the electron beam and most of the other part is a smooth flat surface, it is used for focusing. The difference between the integrated values of the detection signals may be small depending on each focusing state of the electron beam, and the accuracy of focusing may be reduced.

[0004] The present invention has been made in view of such a point, and an object of the present invention is to provide a charged particle beam capable of performing focusing with high accuracy even when a characteristic portion exists only on a part of a sample surface. Is to realize the focusing method in the above.

[0005]

SUMMARY OF THE INVENTION A focusing method in a charged particle beam apparatus according to the present invention scans an irradiation position of a charged particle beam on a sample and a focusing lens for focusing the charged particle beam on the sample. Particle beam apparatus, comprising: scanning means for detecting a signal obtained by irradiating a charged particle beam onto a sample; and means for changing the focusing state of the charged particle beam on the sample in a stepwise manner In each focusing state, a predetermined area of the sample is scanned with the charged particle beam, and at that time, an optimum focus position is obtained based on a signal detected by a detector, and a focusing lens is set at the optimum focus position. A focusing method for performing an operation, wherein a predetermined area of a sample is scanned prior to the focusing operation, a signal obtained as a result is detected, and a predetermined area of the sample is detected. Divided virtually, it compares the integrated value of the signals of each of the divided regions, and characterized in that to perform the focus of the divided regions the integrated value of the maximum intensity was obtained.

[0006]

According to the focusing method in the charged particle beam apparatus according to the present invention, prior to the focusing operation, a region of a characteristic portion of the sample surface is found, and the focusing operation is performed based on a detection signal of the region. .

[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 performing a focusing method according to the present invention, and 1 is an electron gun. Electron beam EB generated from electron gun 1
Is narrowly focused on the sample 4 by the focusing lens 2 and the objective lens 3. Further, the electron beam EB is transmitted to 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 supplied to the high-pass filter 8 and the cathode ray tube 9 after being amplified by the amplifier 7.

The signal passed through the high-pass filter 8 is supplied to an integrator 11 via an absolute value circuit 10. Integrator 1
The integration result of 1 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. An operation panel 15 sends an instruction signal to the control circuit 14. The control circuit 14 controls a drive power supply 16 for the objective lens 3 and a drive power supply 17 for the deflection coil 5. Further, in this embodiment, the signal detected by the detector 6 and amplified by the amplifier 7 is converted into a digital signal by the AD converter 18 and then transmitted to the memory 1 via the control circuit 14.
9 and stored. The operation of such a configuration is as follows.

When a normal secondary electron image is observed, a control circuit 14 controls a driving power supply 17 based on an instruction signal from an operation panel 15, and a predetermined scanning signal is supplied from the driving power supply 17 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 a cathode ray tube 9 synchronized with a scanning signal to the deflection coil 5 via an amplifier 7, and a 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 give an instruction of the focusing mode, the control circuit 14 supplies a drive power supply 16 for the objective lens 3 and a drive power supply 17 for the deflection coil 5.
And control. As a result, the driving power supply 16
To the drive power supply 1
Numeral 7 supplies a scanning signal to the deflection coil 5 for operating a predetermined area of the sample every time the step-like excitation current changes.

A secondary electron signal detected by the detector 6 in a focus state by each step-like exciting current is amplified by an amplifier 7, a DC component is removed by a high-pass filter 8, and an absolute value circuit 10 is converted to a positive signal. The output of the absolute value circuit 10 is supplied to an integrator 11, where a 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 converted to a digital signal.
3 is supplied.

The peak detection circuit 13 stores the integrated value of the integrator 11 for each excitation step of the objective lens 3. FIG. 2 shows the change in the stored integral value at this time, in which the vertical axis represents the integral 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 at the peak of the change curve of the stored integral value,
The value is supplied to the control circuit 14. The control circuit 14
The drive power supply 16 is controlled to set the objective lens 3 to the excitation intensity at the peak, and the focusing operation is performed in this manner.

In the method of the present invention, prior to the above-described focusing operation, an operation of specifying a sample region used for focusing is executed. First, a predetermined region of the sample 4 is scanned with an electron beam, secondary electrons generated based on the scanning are detected by a detector 6, the detection signal is amplified by an amplifier 7, and a digital signal is converted by an AD converter 18. Convert to The converted signal is supplied to the memory 19 via the control circuit 14 and stored.

The control circuit 14 virtually divides the stored secondary electron detection signal of a predetermined region of the sample into regions. FIG. 3 shows the sample image D stored in the memory 19, and a linear feature (for example, I
C pattern) L exists. The surface other than the characteristic portion L is a smooth surface. Such an image area is virtually divided into, for example, eight areas S1 to S8. The dotted line in the figure indicates the boundary of the virtual division. The control circuit 14 integrates the intensity of the image signal stored for each virtual area. As a result, eight types of integrated signals are obtained. The control circuit 14 compares the eight types of integrated signals and finds a region where the maximum integrated signal can be obtained. In the case of FIG. 3, since a linear characteristic portion exists in the region S5, a maximum integrated signal is obtained in the region S5.

According to the above steps, the divided area S5 in which the characteristic portion L exists in the scanning area of the sample 4 is determined. Thereafter, the automatic focusing operation is performed. In this focusing operation, the control circuit 14 controls the drive power supply 17 so that only the region S5 is scanned by the electron beam each time each excitation step of the objective lens 3 is performed. Only the secondary electron signal based on the scanning of the electron beam in the region S5 is amplified by the amplifier 7 and the DC component is removed by the high-pass filter 8, and then converted to a positive signal by the absolute value circuit 10. Absolute value circuit 1
The output of 0 is supplied to the integrator 11, and a 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 calculated by the AD converter 1
After being converted into a digital signal by 2, the signal is supplied to a peak detection circuit 13, and the above-described focusing operation is executed.

In the above-described embodiment, in the automatic focusing operation, only the divided area S5 is scanned by the electron beam. However, the scanning of the electron beam is performed over the entire area, and the area of the detected signal is Only the signal of the portion of S5 may be used for focusing, or the scanning from S1 to S5 may be performed, and the signal of only the region of S5 may be used. Furthermore, although the image area is divided into elongated areas in the horizontal direction, the image area may be elongated in the vertical direction. In addition, as shown in FIG. 4, when there is a characteristic portion in a part P of the image area, the image area is partitioned in both the horizontal and vertical directions, and the signal strength of each divided area may be obtained. good. Then, instead of using only a signal of one specific area as a signal used for auto-focusing, a signal of two areas of an area where the maximum signal strength is obtained and a signal where the second signal strength is obtained are obtained. The signals may be used, or signals in a larger number of regions may be used.

Although one 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 the embodiments, the scanning electron microscope has been described as an example, but the present invention can be applied to an apparatus using an ion beam. Further, although the excitation of the objective lens is changed during the focusing operation, an auxiliary lens for the objective lens may be provided to change the excitation of the auxiliary lens. Furthermore, at the time of focusing, the detection signal is integrated for each scan, and the integrated signal is compared to obtain the optimum focus position. However, the maximum amplitude value (peak-to-peak value) of the detection signal is detected. You may do it.

[0018]

As described above, the focusing method in the charged particle beam apparatus according to the present invention finds a region of a characteristic portion of the sample surface prior to the focusing operation, and outputs a detection signal of the region. Since the focusing operation is performed based on the signal, it is possible to remove the signal from the smooth surface portion and perform the focusing operation. As a result, automatic focusing is performed based on a signal of a characteristic portion on the sample, so that the accuracy of focusing can be improved.

[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 a sample image divided into regions.

FIG. 4 is a diagram showing a sample image divided into regions.

[Explanation of symbols]

 DESCRIPTION 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 A / D converter 19 Memory

──────────────────────────────────────────────────の Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01J 37/21

Claims (1)

(57) [Claims] 1. A focusing lens for focusing a charged particle beam on a sample, a scanning unit for scanning an irradiation position of the charged particle beam on the sample, and a sample obtained by irradiating the sample with the charged particle beam. In a charged particle beam apparatus provided with a detector for detecting a signal and a means for changing a focused state of a charged particle beam on a sample in a step-like manner, a predetermined region of a sample is scanned with a charged particle beam at each focused state. And
At this time, a focusing method for obtaining an optimum focus position based on a signal detected by the detector and performing a focusing operation of setting a focusing lens at the optimum focus position, wherein a predetermined area of the sample is performed prior to the focusing operation. Scanning, detecting the signal obtained as a result, virtually dividing a predetermined region of the sample, comparing the integrated value of the signal for each divided region,
A focusing method in a charged particle beam apparatus, wherein the focusing is performed on a divided region in which the integrated value of the maximum intensity is obtained.