JPH0887973A - Focusing method and apparatus for charged particle beam apparatus - Google Patents

Abstract

PURPOSE: To provide focusing method and apparatus which can carry out focusing at high precision for charged particle beam. CONSTITUTION: Peak values obtained by an integration unit 17 are compared and the focal point at the time when the maximum value is obtained is set to be focus fitted point. At that time, the count numbers np sent from a peak counter 18 are compared and the count numbers of front and back lines of a focus fitting line are the same, the focus fitting position is determined to be effective and in the case the count numbers of the front and back lines are not the same, the obtained focus fitting position is determined to be invalid. In the case the focus fitting position is determined to be invalid, a focus offset for an objective lens 3 is reset and a focus fitting position with which the count numbers of the front and back lines become the same should be found.

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 apparatus in 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 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 the case of a sample such as an IC pattern, there is no unevenness over the entire scanning region of the sample, and there are many linear patterns. FIG. 1 shows an IC pattern, and FIG. 1A shows a vertical IC pattern. The portion indicated by Q in the figure is a region in which an elongated pattern is formed in a convex shape. When focusing is performed on a sample having such a pattern, a predetermined region of the sample is vertically divided into a large number (A to
E region), the focusing state of the electron beam is changed for each divided region. Then, the signals obtained based on the scanning of the electron beam for each of the divided areas are detected and integrated, and the focusing state when the maximum value is obtained is determined to be the focus position, and the focusing lens is set to that state. I am trying to set the excitation.

In such focusing, if the linear pattern of the IC is evenly distributed in the vertical direction as in the case of FIG. 1A, accurate focusing operation can be performed. However, a problem arises when the shape of the pattern changes midway as in the case of FIG. That is, for example, the electron beam is always scanned across the concavo-convex pattern in the A and B areas, but the electron beam does not traverse the concavo-convex portion in the D and E areas, so that the intensity of the obtained signal becomes small. As a result, the integrated value of the detection signal used for focusing changes due to factors other than the focusing state of the electron beam, and the accuracy of focusing decreases.

The present invention has been made in view of the above circumstances, and an object thereof is to realize a focusing method and apparatus for a charged particle beam capable of performing focusing with high accuracy.

[0006]

A focusing method in a charged particle beam apparatus according to the present invention is a method in which a scanning area of a charged particle beam on a sample is divided into a plurality of areas in the vertical direction, and each divided area is scanned with an electron beam. Then, the intensity of the focusing lens for focusing the charged particle beam on the sample is changed stepwise, and the optimum focusing lens intensity is detected based on the signal obtained by scanning each divided area. In the focusing method in the charged particle beam device configured to set the focusing lens intensity, the number of peaks having a predetermined intensity or more among the signals detected by the scanning of the charged particle beam in each divided region is counted, and The feature is that whether the focusing operation is valid or invalid is determined according to the count value.

Further, the focusing device in the charged particle beam system according to the present invention comprises a focusing lens for focusing the charged particle beam on the sample, a means for changing the intensity of the focusing lens in a stepwise manner, and a focusing lens. Each time the lens intensity changes stepwise, the scanning means for moving the irradiation position of the charged particle beam on the sample in the vertical direction to scan, and the signal obtained by the irradiation of the charged particle beam on the sample are detected. In a focusing device in a charged particle beam device comprising a detector and means for setting the intensity of the focusing lens, based on the signal obtained from the detector in response to each step change of the focusing lens intensity, Counting means for counting the number of peaks having a predetermined intensity or more in the detection signal of each scanning region is provided, and focusing is performed based on the count value of the counting means. It is characterized by comprising means for determining the enable or disable of work.

[0008]

In focusing in the charged particle beam apparatus according to the present invention, the number of peaks having a predetermined intensity or more in the signals detected by scanning of the charged particle beam is counted, and focusing is performed according to the count value of each region. Determine whether the operation is valid or invalid.

[0009]

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 finely focused on the sample 4 by the focusing lens 2 and the objective lens (final stage focusing lens) 3. Excitation current is supplied to the objective lens 3 from the computer 5 via the objective lens control unit 6. Also,
The electron beam EB is deflected by the deflector 7, and the sample 4
The irradiation position of the upper electron beam is scanned. A scanning signal is supplied to the deflector 7 from the computer 5 via the electron beam deflection unit 8 and the amplifier 9. A dynamic focus lens 10 is supplied with a dynamic focus lens current from the computer 5 via the dynamic focus lens control unit 11.

Secondary electrons generated by irradiating the sample 4 with the electron beam are detected by the secondary electron detector 12. The detection signal of the detector 12 is amplified by the amplifier 13 and then supplied to the cumulative addition unit 14. The cumulative addition unit 14 performs cumulative addition of the signals detected based on the repeated scanning of the electron beam based on the signal from the electron beam deflection unit 8. The signal from the cumulative addition unit 14 is supplied to the signal differentiating unit 15 to be differentiated. The signal from the signal differentiating unit 15 is supplied to the integrating unit 17 via the absolute value inverting unit 16 to be integrated. The output signal of the integration unit 17 is supplied to the computer 5. Further, the signal from the absolute value inverting unit 16 is supplied to the peak counter 18, and the count value of the peak counter 18 is calculated by the computer 5.
Is supplied to. The operation of such a configuration will be described below.

When observing a normal secondary electron image, the computer 5 controls the electron beam deflection unit 8 and supplies a predetermined two-dimensional scanning signal from the unit 8 to the deflector 7. As a result, 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 12. The detection signal is supplied via an amplifier 13 to a cathode ray tube (not shown) synchronized with the scanning signal to the deflector 5, and a secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube.

Next, the automatic focusing operation of the electron beam will be described. In this automatic focusing, a predetermined region of the sample is vertically divided into a large number, and the focusing state of the electron beam is changed for each divided region. I am letting you. Then, the signals obtained based on the scanning of the electron beam for each of the divided areas are detected and integrated, and the focusing state when the maximum value is obtained is determined to be the focus position, and the focusing lens is set to that state. I am trying to set the excitation. For example, with respect to the sample 5 shown in FIG. 3, eight regions (A to
H).

For the above focusing, the computer 5 controls the dynamic focus lens control unit 11 and the electron beam deflection unit 8. As a result, the dynamic focus lens control unit 11 supplies an exciting current that changes stepwise to the dynamic focus lens 10, and the electron beam deflection unit 8 changes the exciting current in a stepwise manner to eight areas (A to A) of the sample 4. H)
The scanning signal for performing two-dimensional scanning sequentially is supplied to the deflector 7.

2 generated from the sample by scanning sample 4
Secondary electrons are detected by the detector 12. Detector 12
The secondary electron signal detected by is amplified by the amplifier 13 and then supplied to the cumulative addition unit 14.
In this unit 14, the detection signal is stored in the memory address corresponding to the scanning coordinate position of the electron beam based on the reference signal from the electron beam deflection unit 8, and the detection signal based on the scanning of the same area on the sample is stored in the memory. Add up cumulatively on the address. By this operation, the SN of the detection signal
Improve the ratio.

The detection signal cumulatively added by the cumulative addition unit 14 is differentiated by the signal differentiation unit 15. Further, the differential signal is subjected to absolute value inversion processing in the absolute value inversion unit 16. The absolute value inversion processing signal is subjected to integration processing in the integration unit 17. FIG. 4 shows the above-mentioned processing result for the signal detected by the electron beam scanning. FIG. 4A shows a signal that has been detected and cumulatively added.
The signal shown by the solid line in (b) is obtained. The signal subjected to the differential processing is subjected to the absolute value inversion processing in the absolute value inversion unit 16, and by this processing, the negative signal among the differential signals is shown in FIG.
It is inverted as shown by the dotted line. The signal whose absolute value has been inverted is subjected to integration processing in the integration unit 17, and the signal of FIG. 4 (d) is obtained. The final integrated value in FIG. 4D is the peak value P.
Becomes This integration processing is performed on the signal based on the two-dimensional scanning of the electron beam for each excitation step of the objective lens 3. The integrated value of the integration unit 17 is supplied to the computer 5.

The computer 5 stores the integrated value of the integration unit 16 for each excitation step of the dynamic focus lens 10. The computer 5 determines the excitation intensity of the dynamic focus lens when the maximum peak of each integrated value is obtained. Then, the objective lens control unit 6 is controlled, and the objective lens 3 is set to the excitation intensity obtained by adding the excitation intensity of the dynamic focus lens when the maximum peak is obtained, and thus the focusing operation is performed.

In the focusing process described above, the signal subjected to the absolute value inversion processing by the absolute value inversion unit 16 (see FIG. 4).
(B)) is also supplied to the peak counter 18. In the peak counter 18, a predetermined level L is set as shown in FIG. 4B, and for peak values above the level L, a square wave is created as shown in FIG. To count. The count value np of the peak counter 18 is supplied to the computer 5.

Now, as described above, the integration unit 17
The peak values obtained by the integration are compared, and focusing is performed with the focus position when the maximum peak is obtained as the in-focus position. At this time, the count number np sent from the peak counter 18 is compared. If the count numbers np of the lines before and after the in-focus line are the same, the obtained in-focus position is valid, but if the count values of the lines before and after do not match, the obtained in-focus position is obtained. The focal position is invalid. If it is determined to be invalid, the focus offset of the objective lens 3 is reset and the in-focus position where the count numbers np of the preceding and following lines are the same is found.

The above operation will be described with reference to FIG. 4. When the in-focus positions are obtained in the divided areas F to G, the count values of the areas before and after the areas are the same, so that the in-focus positions are obtained. Is valid. On the other hand, when the in-focus position is obtained in the areas B to D, the count values of the areas before and after the area are different, and the in-focus position is invalid. That is, in these regions, the number of patterns traversed by the scanning of the electron beam is different, and the change in this number affects the integral value. In this case, the focus offset of the objective lens 3 is reset so that the focal points are obtained in the areas E to H. In addition, when the count value of the peak counter 18 is 0 and the peak value cannot be obtained, a signal indicating that the measurement is impossible is sent to the computer.

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. Although the excitation of the dynamic focus lens is changed during the focusing operation, the excitation of the objective lens may be changed stepwise without using the dynamic focus lens. Further, in 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. You may do it. Furthermore, although an electron beam apparatus such as a scanning electron microscope has been described as an example, the present invention can be applied to an ion beam apparatus.

[0021]

As described above, in focusing in the charged particle beam apparatus according to the present invention, the number of peaks having a predetermined intensity or more among the signals detected by the scanning of the charged particle beam is counted and each region is counted. It is configured to determine whether the focusing operation is valid or invalid according to the count value of. As a result, it is possible to perform an accurate focusing operation even if regions having different patterns exist in the scanning region of the charged particle beam.

[Brief description of drawings]

FIG. 1 is a diagram showing an IC pattern.

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

FIG. 3 is a diagram showing how a scan area of a sample is divided for automatic focusing.

FIG. 4 is a diagram showing operation signal waveforms in the embodiment of FIG.

[Explanation of symbols]

 1 Electron Gun 2 Focusing Lens 3 Objective Lens 4 Sample 5 Computer 6 Objective Lens Control Unit 7 Deflector 8 Electron Beam Deflection Unit 9 Amplifier 10 Dynamic Focus Lens 11 Dynamic Focus Lens Unit 12 Secondary Electron Detector 13 Amplifier 14 Cumulative Addition Unit 15 Signal differentiation unit 16 Absolute value inversion unit 17 Integration unit 18 Peak counter

Claims (2)

[Claims] 1. A step of increasing the intensity of a focusing lens for focusing a charged particle beam onto a sample by dividing a charged particle beam scanning region on the sample into a plurality of parts in the vertical direction and scanning the electron beam in each divided region. The focusing method in the charged particle beam apparatus, in which the intensity of the optimum focusing lens is detected based on the signal obtained by scanning each divided region and the focusing lens intensity is set to that intensity. In the above, the number of peaks having a predetermined intensity or more among the signals detected by the scanning of the charged particle beam in each divided region is counted, and whether the focusing operation is valid or invalid is determined according to the count value of each region. A method for focusing in a charged particle beam device, comprising: 2. A focusing lens for focusing the charged particle beam on the sample, a means for changing the intensity of the focusing lens in a stepwise manner, and a charge on the sample for each stepwise change of the focusing lens intensity. Scanning means for moving the particle beam irradiation position in the vertical direction for scanning, a detector for detecting the signal obtained by irradiating the sample with the charged particle beam, and a stepwise change of the focusing lens intensity. In the focusing device of the charged particle beam device, which comprises means for setting the intensity of the focusing lens, based on the signal obtained from the detector, the number of peaks having a predetermined intensity or more in the detection signal is counted. A charged particle beam, comprising: counting means, and means for determining whether the focusing operation is valid or invalid based on the count value of the counting means. Focusing device in the device.