JPH1173903A - Autofocus method for scanning electron microscope - Google Patents

Abstract

(57) [Summary] [Problem] To provide an automatic focusing method of a scanning electron microscope that can perform an automatic focusing operation of a structural portion of a sample to be observed with high accuracy even when the structure of the sample on an observation screen is biased. Realize. SOLUTION: After executing autofocus using the entire screen, a memory 31 stores a detection signal of the entire screen after execution of primary autofocus. The detection signal stored in the memory 31 is supplied to a data integration unit 32, where the detection signal is integrated for each divided area. If the autofocus is executed again in the area where the numerical value of the integrated value for each area is large, it becomes easier to obtain a change in the signal image. For this reason, the integrated value for each divided area is compared by the data comparison unit 33, and for example, the area where the maximum integrated value is obtained is selected. The data comparison unit 33 controls the vertical scanning signal generation circuit 7 and the horizontal scanning signal generation circuit 9, and sets the scanning region of the electron beam 1 as a selected region. The secondary autofocus is executed with the scanning area limited.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus method for a scanning electron microscope, which can perform autofocus of an electron beam with high accuracy.

[0002]

2. Description of the Related Art FIG. 1 shows an example of a conventional scanning electron microscope having an autofocus function. Reference numeral 1 denotes an accelerated electron beam generated from an electron gun (not shown).
Reference numerals 2 and 3 denote two-stage deflection coils.
Include horizontal and vertical deflection coils. Reference numeral 4 denotes an objective lens, and the electron beam finely focused by the objective lens 4 irradiates a sample 5. 6 is generated by irradiating the sample 5 with an electron beam, for example, 2
This is a detector for detecting the next electron.

A horizontal scanning signal is supplied from a horizontal scanning signal generation circuit 7 via a driving circuit 8 to horizontal deflection coils of the two-stage deflection coils 2 and 3, and a vertical scanning signal generation circuit is supplied to a vertical deflection coil. A vertical scanning signal is supplied from a circuit 9 via a driving circuit 10. Speed (period) of the scanning signal from the horizontal scanning signal generation circuit 7 and the vertical scanning signal generation circuit 9
Is controlled by a control circuit 11 such as a computer.

[0004] The signal detected by the detector 6 is:
The horizontal and vertical scanning signals are supplied to a cathode ray tube 13 and a filter circuit 14 through an amplifier 12. The output signal of the filter circuit 14 is supplied to the integration circuit 16 via the absolute value circuit 15 and integrated by the integration circuit 16. The integrated value of the integration circuit 16 is converted into a digital signal by the AD converter 17 and stored in the signal intensity distribution memory 19 in the control circuit 11.

The control circuit 11 has a maximum value detection unit 20 for detecting the maximum value of a large number of integrated values stored in the signal intensity distribution memory 19. Further, the control circuit 11
Has an objective lens value setting data memory 21 and an auxiliary coil value setting data memory 22. The value of the objective lens value setting data memory 21 is supplied to an objective lens drive circuit 24 via a DA converter 23.

Also, an auxiliary coil value setting data memory 2
The value of 2 is supplied to the drive circuit 27 of the auxiliary coil 26 via the DA converter 25. Reference numeral 28 denotes an objective lens value conversion unit, and 29 denotes an autofocus unit. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the control circuit 11 controls the horizontal scanning signal generation circuit 7 and the vertical scanning signal generation circuit 9 so that a desired scanning speed is obtained. As a result, a desired deflection signal is generated. The electron beam 1 supplied to the deflection coils 2 and 3 is deflected by the deflection coils 2 and 3 and scans a desired area of the sample 5.

[0008] Secondary electrons generated by the irradiation of the sample 5 with the electron beam are detected by a detector 6, and the detection signal is amplified by an amplifier 12 and then supplied to a cathode ray tube to which a scanning signal is supplied. 13, a secondary electron image of the sample is displayed on the cathode ray tube 13.

Next, FIG.
A description will be given with reference to an operation curve of the auxiliary coil 26 in FIG. 2A and an operation curve of the objective lens 4 in FIG. 2A and 2B, the horizontal axis represents time, and the vertical axis represents excitation intensity (equal to the distance between the sample surface and the lens).

First, an objective lens value setting data memory 2
In FIG. 1, an initial value Zo shown in FIG. 2B is set, and the objective lens driving circuit 24 is operated based on the initial value to excite the objective lens 4. Next, the auto focus start unit 29 is operated to change the value of the auxiliary coil value setting data memory 22 as shown in FIG. FIG.
In FIG. 3A, the change of the set value is shown as a straight line for convenience, but specifically, the change is made in a step-like manner as shown in FIG.

Each time this step change occurs, a scanning signal for scanning a desired area on the sample 5 once is supplied to the deflection coils 2 and 3. Secondary electrons generated based on the irradiation of the sample 5 with the electron beam are detected by the detector 6. The detection signal is amplified by the amplifier 12, and after a specific frequency component is cut by the filter circuit 14,
In the absolute value circuit 15, the negative signal is inverted.

The output of the absolute value circuit 15 is supplied to an integrating circuit 16 where the signal is integrated. This integration is performed during one two-dimensional scan of the desired area on the sample, and after the scan is completed, the integrated value is sent to the signal intensity distribution memory 19 in the control circuit 11 via the AD converter 17. Is memorized. When such an integration operation is performed for each step-like excitation change of the auxiliary coil 26 and from + A to -A,
The distribution shown in FIG. 3B is stored in the memory 19.

The maximum value detection unit 20 in the control circuit 11
Detects the maximum value of the distribution of FIG. 3B and determines the excitation intensity to the auxiliary coil 26 at that time by the objective lens value conversion unit 2.
8 Note that the electron beam is in focus at the time of this maximum value excitation. As a result, the value of the objective lens value setting data memory 21 is a value obtained by adding the excitation intensity of the auxiliary coil (the amount of deviation between the initial value Zo and the focal point position) ΔZ1 when the focus is on the initial setting value Zo. Becomes

After setting the excitation of the objective lens 4 to Zo + ΔZ1, the above-described autofocus operation is executed again,
The shift amount ΔZ2 between Zo + ΔZ1 and the focal point value at that time is set in the auxiliary coil value setting data memory 29. The excitation value ΔZ1 of the set objective lens and the excitation value ΔZ2 of the auxiliary coil are shown in FIGS. 2 (a) and 2 (b). When the setting of ΔZ1 and ΔZ2 is completed, the auto focus operation ends.

[0015]

In the above-described autofocus operation, the focus of the electron beam is changed on the sample, and the in-focus position is detected based on a change in the signal of the entire image displayed on the observation screen. ing. However, the structure of the sample displayed on the observation screen may be biased as a whole.

For example, FIG. 4 shows an example of the observation screen. In the example of FIG. 4, a structure R having relatively severe irregularities is distributed in the upper right area of the observation screen D, and the other areas are flat. Has become. In such a case, it is desired to focus the electron beam in the region of the structure R. However, since a detection signal in a flat portion having no structure is also taken in as information, the amount of information in the region to be focused is efficiently reduced. Can not be used well. As a result, the image information at the time of autofocus cannot be used efficiently, which affects accuracy for focusing.

The present invention has been made in view of such a point, and an object of the present invention is to perform an operation such as auto-focusing of a structural portion of a sample to be observed even when the structure of the sample on an observation screen is uneven. An object of the present invention is to realize an autofocus method of a scanning electron microscope that can be performed with high accuracy.

[0018]

According to a first aspect of the present invention, there is provided an autofocusing method for a scanning electron microscope, in which a focus state of an electron beam on a sample is changed stepwise, and an electron beam in a specific region on the sample at each stage is changed. In the auto-focusing method of the scanning electron microscope, the detection signal obtained based on the beam scanning is integrated, and the electron beam is controlled to a focus state where the maximum integrated value is obtained by comparing the integrated values. After performing the primary autofocus on the entire specified area, the specified area is divided into a plurality of areas, detection signals obtained by electron beam scanning for each of the divided areas are integrated, and division is performed according to the integrated value. An area is selected, and a secondary autofocus operation is performed in the selected divided area.

In the first invention, after performing the primary autofocus of the entire specified area, the specified area is divided into a plurality of areas, and detection signals obtained by scanning the electron beam for each of the divided areas are integrated. A divided area having a large integrated value or a divided area in which the integrated value is within a specific range is selected, and a secondary autofocus operation is performed on the selected divided area.

According to a second aspect of the present invention, in the autofocus method for a scanning electron microscope according to the first aspect, detection signals obtained by scanning the electron beam for each of the divided areas are integrated, and the integrated value is a predetermined threshold. It is characterized in that a divided region having a value equal to or larger than the value is selected, and a secondary autofocus operation is executed in the selected divided region.

In the second invention, a divided area equal to or larger than a specific threshold value is selected. The autofocus method for a scanning electron microscope according to the third invention is the method according to the first invention, wherein detection signals obtained by scanning the electron beam for each of the divided areas are integrated, and an average value of each integrated value is obtained. It is characterized in that a range of a threshold value is determined in accordance with the average value, a divided region of the integrated value within the range of the threshold value is selected, and a secondary autofocus operation is performed in the selected divided region.

In the third invention, the average value of the integrated value of each divided area is obtained, the range of the threshold value is determined according to the average value, and the divided area of the integrated value within the range of the threshold value is selected. .
An autofocus method for a scanning electron microscope according to a fourth aspect of the present invention is obtained by changing the state of focus of an electron beam on a sample in a stepwise manner, based on scanning of a specific region on the sample in each stage with the electron beam. In a scanning electron microscope autofocusing method in which a detection signal is integrated and an electron beam is controlled to a focus state where a maximum integrated value is obtained by comparing the integrated values, a primary autofocus of the entire specific area is performed. Is performed, the specific area is divided into a plurality of areas, the contrast value of each divided area is obtained from the detection signal obtained by scanning the electron beam for each divided area, and the divided area having a large contrast value is selected. The secondary auto focus operation is performed in the selected divided area.

In the fourth aspect, after the autofocus of the entire specific area is performed, the specific area is divided into a plurality of areas, and each of the divided areas is obtained from a detection signal obtained by scanning the electron beam for each of the divided areas. A contrast value is obtained, a divided area is selected according to the contrast value, and a secondary autofocus operation is performed on the selected divided area.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 5 shows an example of a scanning electron microscope for implementing the autofocus method according to the present invention. Elements that are the same as or similar to those in FIG. 1 are denoted by the same reference numerals, and detailed description is omitted. I do.

In the configuration shown in FIG. 5, the secondary electrons from the sample 5 are detected by the secondary electron detector 6, and this detection signal is supplied to the amplifier 12 and the filter circuit 1.
4. The signal is supplied to the AD converter 17 via the absolute value circuit 15.

The detection signal converted into a digital signal by the AD converter 17 is stored in a memory 31 in the control circuit 11. The signal stored in the memory 31 is subjected to signal integration in a data integration unit 32.

The signal integrated by the data integration unit 32 is supplied to the signal intensity distribution memory 19 or the data comparison unit 33. The data comparison unit 33 compares the data and controls the vertical scanning signal generation circuit 7 and the horizontal scanning signal generation circuit 9. The operation of such a configuration will now be described.

First, the same operation as the auto focus using the entire observation screen described with reference to FIGS. 1 to 3 is executed. That is, the objective lens value setting data memory 21
The initial value Zo shown in FIG. 2 (b) is set, and the objective lens driving circuit 24 is operated based on the initial value to excite the objective lens 4. Next, the auto focus start unit 29 is operated, and the auxiliary coil value setting data memory 2 is stored.
2 is changed as shown in FIG.

Each time the step change occurs, a scanning signal for scanning a desired area on the sample 5 once is supplied to the deflection coils 2 and 3. Secondary electrons generated based on the irradiation of the sample 5 with the electron beam are detected by the detector 6. The detection signal is amplified by the amplifier 12, and after a specific frequency component is cut by the filter circuit 14,
In the absolute value circuit 15, the negative signal is inverted.

The output of the absolute value circuit 15 is converted into a digital signal by the AD converter 17 and then supplied to the memory 31 for storage. The signals stored in the memory 31 are integrated by a data integration unit 32 with signals for one screen. This integrated value is sent to and stored in the signal intensity distribution memory 19 in the control circuit 11. When such an integration operation is performed for each step-like excitation change of the auxiliary coil 26 and from + A to -A, the distribution shown in FIG. 3B is stored in the memory 19.

The maximum value detection unit 20 in the control circuit 11
Detects the maximum value of the distribution of FIG. 3B and determines the excitation intensity to the auxiliary coil 26 at that time by the objective lens value conversion unit 2.
8 Note that the electron beam is in focus at the time of this maximum value excitation. As a result, the value of the objective lens value setting data memory 21 is a value obtained by adding the excitation intensity of the auxiliary coil (the amount of deviation between the initial value Zo and the focal point position) ΔZ1 when the focus is on the initial setting value Zo. Becomes

After performing the primary autofocus using the entire screen, the memory 31 stores a detection signal of the entire screen after the primary autofocus is performed. The area of one screen corresponds to, for example, 9) of a) to i) as shown in FIG. 6 with respect to the sample having the structure shown in FIG.
Is virtually divided into two areas. The detection signal stored in the memory 31 for each divided area is output to the data integration unit 3
2 and integrated by this unit.

FIG. 7 shows the integration result for each divided area. As is clear from this figure, the integrated value in the region of the structure with severe irregularities on the sample surface has a large value, and conversely, the integrated value in the region of the flat structure has a small value. That is, the integrated value of the region c) having a large unevenness is a large value of 700, and the flat regions a), d), h) and i) are 100.
And a small value.

The integrated value for each region indicates the presence or absence of a structure on the sample. If autofocus is performed again in a region having a large numerical value (a region having more structures on the sample), a signal image can be further improved. Changes easily.

Therefore, in this embodiment, the integrated value for each divided area is compared by the data comparing unit 33, and for example, the area c) where the maximum integrated value is obtained is selected. Based on this selection, the data comparison unit 33 controls the vertical scanning signal generation circuit 7 and the horizontal scanning signal generation circuit 9 to set the scanning region of the electron beam 1 to the region c).

With the scanning area limited, the same autofocus operation (secondary autofocus) as described above is performed. In this case, as shown in FIG. 8A, the step-like excitation change to the auxiliary coil 26 has a narrower range than in the case of the primary autofocus. In addition,
FIG. 8B shows an operation curve of the objective lens 4.

As described above, after the primary and secondary autofocus are executed, the entire screen is observed in a focus state based on the result of the secondary autofocus.
Therefore, it is possible to observe an image in a state where the focus is optimally focused on a region having a structure on the observation screen of the sample.

In the above-described embodiment, when selecting an area in which the secondary autofocus is to be executed from among the divided areas, the area where the maximum integrated value is obtained is selected. However, another selection method is used. Is also good. For example, a threshold value may be set in advance, and an area having an integrated value equal to or larger than the threshold value may be selected.

For example, in the example of FIG. 7, the threshold value may be set to 400, and the secondary autofocus may be executed in three areas b), c) and f) above the threshold value. Alternatively, an upper limit and a lower limit of the threshold value may be set, and a region in which the integrated value is distributed therebetween may be selected.

As an example, when the threshold value is ± 10% of the average value of the integrated values of all the divided areas, in the screen division example shown in FIG. 9, the average value is 510, and the average value is + 10%.
The threshold value is 561 which is%, and 459 which is -10% of the average value. In the example of FIG. 9, since the integrated values of the nine divided regions are all within the threshold value, the entire screen is used in the secondary autofocus.

In the above-described embodiment, the entire screen is divided, the integrated value for each divided area is obtained, and the integrated value is compared to select the area where the secondary autofocus operation is performed. In addition to the method of comparing the contrast values, the contrast values of the respective divided regions may be compared.

FIG. 10 shows a partial configuration of a scanning electron microscope when the method of comparing the contrast values is performed. 10, the detection signal converted into a digital signal by the AD converter 17 is supplied to a memory 31 in the control circuit 11 and stored.

The signal stored in the memory 31 is supplied to a data integrating unit 32 and also to a contrast value detecting unit 34. Data integration unit 3
The value integrated by 2 is supplied to the signal intensity distribution memory 19.

The contrast value detection unit 34 detects the contrast value of the detection signal stored in the memory 31, that is, the peak-to-peak value of the signal. The detection of the contrast value is performed for each divided region obtained by dividing the entire screen. The contrast value for each divided area is supplied to the data comparison unit 35.

Here, the detected contrast value is large in a region corresponding to the observation screen where a structure with severe irregularities exists on the sample, and small in a flat sample structure. As a result, the data comparison unit 35 selects a divided region having the maximum contrast value, a divided region having a specific threshold value or more, or a divided region included in the range of the specific threshold value.

Although not shown, the data comparison unit 35 controls the vertical scanning signal generation circuit 7 and the horizontal scanning signal generation circuit 9 shown in FIG.
Is performed. A secondary autofocus operation is performed based on a signal detected by scanning the electron beam in the selected area.

Although the embodiments of the present invention have been described in detail, the present invention is not limited to the above embodiments. For example, although secondary electrons are detected, reflected electrons may be detected. In addition, although the step-like focus change is performed using the auxiliary lens, the step-like focus change may be performed using the objective lens without using the auxiliary lens.

[0048]

As described above, in the first to third aspects of the present invention, after performing the primary autofocus of the entire specific area, the specific area is divided into a plurality of areas, and the electron beam of each of the divided areas is divided. Integrating the detection signals obtained by scanning,
Since a divided area with a large integrated value or a divided area in which the integrated value is within a specific range is selected and the secondary autofocus operation is performed in the selected divided area, the structure is used in the observation area of the sample. Even if there is a bias, the sample image can be observed in a focus state that is optimally matched to a region where the structure changes drastically.

According to the fourth aspect of the present invention, after performing the autofocus for the entire specified area, the specified area is divided into a plurality of areas, and the detection signal of each divided area is used to detect each divided area. Since the contrast value is obtained, a divided area is selected according to the contrast value, and the secondary autofocus operation is performed in the selected divided area, even if the structure is biased in the observation area of the sample,
Observation of the sample image can be performed in a focus state that optimally matches a region where the structure changes drastically.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional scanning electron microscope having an autofocus function.

FIG. 2 is a signal wave diagram used for explaining an autofocus operation by the scanning electron microscope of FIG.

FIG. 3 is a signal wave diagram used for explaining an autofocus operation by the scanning electron microscope of FIG. 1;

FIG. 4 is a diagram showing an example in which the structure of the sample surface is biased.

FIG. 5 is a diagram showing an example of a scanning electron microscope for performing the method of the present invention.

FIG. 6 is a diagram showing a state of division of an observation screen.

FIG. 7 is a diagram showing an integrated value of each divided area.

FIG. 8 is a diagram showing a state of excitation of an auxiliary lens and an objective lens in the method of the present invention.

FIG. 9 is a diagram showing an integrated value of each divided area.

FIG. 10 is a diagram showing a partial configuration of a scanning electron microscope for implementing another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 electron beam 4 objective lens 5 sample 6 detector 11 control circuit 14 filter circuit 15 absolute value circuit 17 AD converter 20 maximum value detection unit 21 objective lens value setting data memory 23, 25 DA converter 28 objective lens value conversion unit 29 Auto focus unit 31 Memory 32 Data integration unit 33 Data comparison unit

Claims (4)

[Claims] 1. A focus state of an electron beam on a sample is changed stepwise, detection signals obtained based on electron beam scanning of a specific area on the sample at each stage are integrated, and the integrated values are compared. In the autofocus method of the scanning electron microscope in which the electron beam is controlled to the focus state where the maximum integrated value is obtained by performing the primary autofocus of the entire specific area,
The specific area is divided into a plurality of areas, detection signals obtained by scanning the electron beam for each of the divided areas are integrated, a divided area is selected according to the integrated value, and a secondary auto-selection is performed on the selected divided area. An autofocus method for a scanning electron microscope that performs a focus operation. 2. A method of integrating detection signals obtained by scanning an electron beam for each divided area, selecting a divided area having an integrated value equal to or greater than a predetermined threshold value, and performing secondary autofocus on the selected divided area. 2. The method according to claim 1, wherein the operation is performed. 3. A detection signal obtained by scanning an electron beam for each divided region is integrated, an average value of each integrated value is obtained, and a threshold value range is determined according to the average value. 2. The autofocus method for a scanning electron microscope according to claim 1, wherein a divided area of the integrated value within the value range is selected, and a secondary autofocus operation is performed in the selected divided area. 4. A method of changing a focus state of an electron beam on a sample in a stepwise manner, integrating detection signals obtained based on electron beam scanning of a specific region on the sample in each stage, and comparing the integrated values. In the autofocus method of the scanning electron microscope in which the electron beam is controlled to the focus state where the maximum integrated value is obtained by performing the primary autofocus of the entire specific area,
The specific region is divided into a plurality of regions, the contrast value of each divided region is obtained from a detection signal obtained by scanning the electron beam for each divided region, a divided region having a large contrast value is selected, and the selected divided region is selected. An autofocus method for a scanning electron microscope in which a secondary autofocus operation is performed in an area.