JPH1050245A - Focusing method in charged particle beam device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make accurate automatic focusing possible without producing the omission of a necessary wave form in automatic focusing operation, and capable of keeping the distinctive feature of a sample within the scanning range of an electron beam even if a scanning distance is narrowed. SOLUTION: As the pre-stage of focusing operation, magnification for focusing is assigned in a computer 5. The computer 5 calculates the distance S1 of the actual spot by the magnification. The maximum effective scanning distance S2 is previously set in the computer 5. The maximum effective scanning distance S2 is the distance at which the omission of wave form information is not produced. The computer 5 compares two kinds of distances S1 and S2 , and if S1 is larger than S2 it converts the distance of the scanning spot in the scanning direction of electron beams from S1 to S2 , and makes the magnification in the scanning direction of electron beams high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a focusing method for a charged particle beam apparatus such as a scanning electron microscope which can automatically focus 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.

In the case of a sample such as a semiconductor device, an electron beam is scanned in a direction perpendicular to the continuous direction (intersecting perpendicularly to the pattern) with respect to a continuous pattern in the x direction or the y direction. .

[0004]

In the above focusing, the electron beam is digitally scanned under the condition of a predetermined magnification. FIG. 1 shows the relationship between the sample image and the scanning spot of the electron beam when the magnification is low. A scanning image (corresponding to the scanning range of the electron beam on the sample) is shown in the upper part of FIG. The lower part shows the scanning spot of the electron beam. The portion indicated by P in the scanned image in FIG. 1 is a characteristic portion of the sample, and a portion where a large change in the secondary electron signal is detected.

[0005] When the magnification is low, the electron beam is scanned over a relatively wide area of the sample, for example, in the horizontal direction in the figure at 512 pixels. In this case, as shown in the lower part of FIG. 1, the interval between the scanning spots of the electron beam becomes relatively long.

FIG. 2 shows the relationship between the sample image and the scanning spot of the electron beam when the magnification is high. The scanning image is shown in the upper part of FIG. 2, and the scanning spot of the electron beam is shown in the lower part of FIG. Is shown. The portion indicated by P in the scanning image in FIG. 2 is a characteristic portion of the sample, and a large 2
This is the part where the change in the next electron signal is detected.

[0007] When the magnification is high, the electron beam is scanned over a relatively wide area of the sample, for example, in the horizontal direction in the drawing at 512 pixels. In this case, as shown in the lower part of FIG. 2, the interval between the scanning spots of the electron beam is relatively short.

A comparison between the case where the magnification is low and the case where the magnification is high indicates that when the magnification is low, the peak profile of the secondary electron detection signal obtained by electron beam scanning has a coarse sampling interval during electron beam scanning. In addition, waveform information drops as compared with the case where the magnification is high.

When the waveform information drops, the shape of the pattern on the sample relative to the focus value of the objective lens at the time of focusing does not appear effectively, and a peak profile may not be obtained properly. As a result, there may be cases where automatic focusing fails.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to realize a focusing method for a charged particle beam that can always automatically perform automatic focusing.

[0011]

According to a first aspect of the present invention, there is provided a focusing method in a charged particle beam apparatus, wherein the intensity of a focusing lens for focusing a charged particle beam on a sample is changed stepwise, and the change is performed. Each time, the irradiation position of the charged particle beam on the sample is scanned, the signal obtained by this scanning is differentiated, the differentiated signal is integrated, and the focusing lens is adjusted to the intensity at which the maximum integrated signal was obtained. In the focusing method that is set, the maximum effective scanning interval of the charged particle beam is set in advance, and the maximum effective scanning interval is compared with the actual scanning interval based on the set magnification. When the interval is longer than the interval, at least the magnification in the scanning direction of the charged particle beam is increased to set the actual scanning interval to be equal to or less than the maximum effective scanning interval.

In the focusing in the charged particle beam apparatus according to the first aspect of the present invention, the maximum effective scanning interval of the charged particle beam is set in advance, and the maximum effective scanning interval is compared with the actual scanning interval based on the set magnification. When the actual scanning interval is longer than the maximum effective scanning interval, at least the magnification in the scanning direction of the charged particle beam is increased so that the actual scanning interval is shorter than the maximum effective scanning interval.

According to a second aspect of the present invention, there is provided a focusing method in a charged particle beam apparatus, wherein a scanning position at which a maximum value of a differential signal is obtained is detected, and a scanning range of the charged particle beam when the scanning position is increased in magnification. It is characterized by performing an image shift so as to enter.

In the focusing in the charged particle beam apparatus according to the second aspect of the present invention, a scanning position at which the maximum value of the differential signal is obtained is detected, and an image shift is performed based on the scanning position. Was set to fall within the scanning range of the charged particle beam when the value of was increased.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 3 shows an example of a scanning electron microscope according to the present invention. In the figure, reference numeral 1 denotes an electron gun. An electron beam EB generated from the electron gun 1 is focused on a focusing lens 2 and an objective lens (final stage focusing lens) 3 Is focused on the sample 4 finely.

An exciting current is supplied from the computer 5 to the objective lens 3 via the objective lens control unit 6.
The electron beam EB is deflected by the deflector 7,
The irradiation position of the electron beam on the sample 4 is scanned. The deflector 7 has an electron beam deflection unit 8 from the computer 5,
The scanning signal is supplied via the amplifier 9. Reference numeral 10 denotes a dynamic focus lens.
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 an electron beam are detected by a secondary electron detector 12. The detection signal of the detector 12 is supplied to a cumulative addition unit 14 after being amplified by an amplifier 13. The accumulative addition unit 14 stores a detection signal at a 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 outputs the detection signal based on the scanning of the same linear area on the sample. Cumulative addition is performed on the memory address.

The output signal from the cumulative addition unit 14 is
The signal is supplied to the signal differentiating unit 15 to perform a differentiating process. The signal from the signal differentiating unit 15 is supplied to an integrating unit 17 via an absolute value inverting unit 16 and is integrated. The output signal of the integration unit 17 is a computer 5
Supplied to

The output of the absolute value reversing unit 16 is supplied to a peak profile storage unit 18 which stores the peak profiles in the X and Y directions. The peak profile stored in the peak profile storage unit 18 is supplied to a waveform change check unit 19, where a waveform change described later is checked.

The waveform change check unit 19 stores the coordinates of the maximum value of the absolute value inverted differential signal (point having the maximum change amount of the input signal). The coordinate value data is supplied to the image shift unit 20. The image shift unit 20 supplies an image shift signal to the electron beam deflection unit 8 based on the supplied coordinate value data. The operation of such a configuration will now be described.

When observing a normal secondary electron image, the computer 5 controls the electron beam deflecting 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 the irradiation of 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 a 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. First, the sample 4 is set at an arbitrary observation position in order to perform automatic focusing. Then, when the focusing mode is instructed to the computer 5, the computer 5
Sets a reference lens value to 3 for the objective lens via the objective lens control unit 6. Further, the computer 5 sets a dynamic focus value for data sampling on the dynamic focus lens 10 via the dynamic focus lens control unit 11.

Next, the computer 5 causes the electron beam deflection unit 8 to generate a scanning signal for linearly scanning the electron beam on the sample 4. This scanning signal is supplied to the deflector 7 via the amplifier 9. As a result, for example, an electron beam is repeatedly scanned in the x direction on a pattern linearly arranged in the y direction on the sample 4. Secondary electrons are generated from the sample 4 based on the scanning of the electron beam on the sample 4. The generated secondary electrons are detected by the secondary electron detector 12.

The detection signal of the secondary electron detector 12 is supplied to the amplifier 1
After being amplified by 3, it is supplied to a cumulative addition unit 14. The accumulative addition unit 14 stores a detection signal at a 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 outputs the detection signal based on the scanning of the same linear area on the sample. Cumulative addition is performed on the memory address. This operation improves the SN ratio of the detection signal. FIG. 4A shows a detection signal waveform obtained by cumulative addition.

The detection signals obtained by the cumulative addition in the cumulative addition unit 14 are differentiated in a signal differentiating unit 15. Further, the differential signal is subjected to absolute value inversion processing in an absolute value inversion unit 16. The solid line in FIG. 4B indicates the differentiated signal, and the dotted line indicates the signal subjected to the absolute value inversion processing. The signal subjected to the absolute value inversion processing is subjected to integration processing in an integration unit 17. FIG. 4C shows the integration signal, and the peak value which is the final integration value is supplied from the integration unit 17 to the computer 5.

The above series of signal processing is performed by the computer 5 by the dynamic focus lens control unit 1.
1 and the electron beam deflection unit 8 are repeatedly executed. That is, the dynamic focus lens control unit 11 supplies a stepwise exciting current to the dynamic focus lens 10, and the electron beam deflecting unit 8 performs a linear scan of a predetermined region of the sample every time the stepwise exciting current changes. A scanning signal to be performed is supplied to the deflector 7.

As a result, the secondary electron signal detected by the detector 12 in the focus state by each step-like excitation current is amplified by the amplifier 13 and then
The signal is supplied to the signal differentiating unit 15. After the negative signal of the differentiated signal is inverted by the absolute value inverting unit 16, the integrating process is performed by the integrating unit 17. Therefore,
The computer 5 includes a dynamic focus lens 1
The integration value of the integration unit 16 for each excitation step of 0 is stored.

The computer 5 controls the objective lens control unit 8, compares the supplied integral values for each excitation step, and adds the excitation intensity of the dynamic focus lens when the maximum integral value is obtained. The objective lens 3 is set to the intensity, and the focusing operation is performed in this manner.

As a pre-stage of the focusing operation, the computer 5 is designated with a magnification for focusing. Computer 5 calculates the distance S ₁ of the actual scanning spot than this ratio. On the other hand, computer 5
The maximum effective scanning intervals S ₂ beforehand is set to. This maximum effective scanning interval S ₂ is the interval there is no fall of the waveform information.

The computer 5 compares the two types of intervals S ₁ and S _2, and if S ₁ is equal to or less than S ₂ , immediately executes the series of automatic focusing operations described above. Conversely, S ₁ becomes S ₂
If more than, the spacing of the scanning direction of the scanning spot of the electron beam is changed from S ₁ to S _2, the magnification of the scanning direction of the electron beam with high magnification. FIG. 5 illustrates this operation.
5 (a) is the scanning width D ₁ of the horizontal direction (X direction) in FIG. 5
(B) to be a scanning width D ₂ narrower as shown, a series of automatic focusing operation described above is executed.

By such an operation, the interval between the scanning spots of the electron beam is set to an interval at which the waveform information does not drop, so that accurate automatic focusing can be executed. FIG. 6 shows a case where S ₁ is equal to or larger than S ₂ , and the scanning width is narrowed (increased in magnification) in both the X direction and the Y direction.
(A) shows a scanning area in front of the electron beam to adjust the scanning width, FIG. 6 (b) X direction, the scan width in the Y direction both show the scanning area narrowed to D _1. Even in this case, since the interval between the scanning spots of the electron beam is set to an interval where the waveform information does not drop, accurate automatic focusing can be performed.

FIG. 7 is a diagram for explaining another embodiment of the present invention. First, a series of automatic focusing operations described above are executed at a set magnification. The absolute value inverted differential signal obtained by this operation is supplied to the storage unit 18 that stores the peak profile in the line direction. The peak profile stored in the storage unit 18 is supplied to a waveform change check unit 19, and this unit 19
, The waveform maximum change coordinates are obtained. For example, FIG.
In the differential signal in which the absolute value is inverted in (b), the waveform maximum change coordinate is point Q.

The waveform maximum change coordinates are supplied to the image shift unit 20. The image shift unit 20 sends an image shift signal to the deflection unit 8 so that the supplied coordinates move to the center position of the screen of the scanned image. Supply. As a result, a signal obtained by adding the image shift signal to the deflection signal is supplied from the deflection unit 8 to the deflector 7, and the characteristic region of the sample is located at the center of the scanning region of the electron beam.

After that, the computer 5 sets two intervals S _1.
And compared with the S _2, S ₁ executes a series of automatic focusing operation immediately above if S ₂ or less. Conversely, S
If ₁ is S ₂ or more, the interval between the scanning direction of the scanning spot of the electron beam is changed from S ₁ to S _2, the magnification of the scanning direction of the electron beam with high magnification. FIG. 7A shows a scanning region of the electron beam before starting the above-described series of operations,
7 (b) is the image shift is executed, further, shows a scanning region D ₂ of the electron beam after the magnification of the X-direction is increased.

In this embodiment, since the characteristic portion on the sample is always located at the center of the scanning area of the electron beam,
At the time of the automatic focusing operation, a change in the integral value at each excitation step becomes large, and more accurate focusing can be performed. Note that the waveform maximum change coordinate does not necessarily need to be located at the center of the scanning range, and it is sufficient that this coordinate is included in the scanning range of the electron beam.

Although the embodiments of the present invention have been described in detail, the present invention is not limited to these embodiments. For example, although secondary electrons are detected, reflected electrons may be detected. In addition, although the case where the electron beam is scanned perpendicularly to the continuous pattern has been described, the scanning may be performed on a sample having irregular irregularities.

Further, 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. Furthermore, although the integration processing is performed after differentiating and inverting the absolute value of the detection signal, the detection signal itself may be integrated.
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.

In the above embodiments, the scanning interval of the electron beam is the maximum effective scanning interval. However, the scanning interval of the electron beam may be smaller than the maximum effective scanning interval.

[0039]

As described above, according to the first aspect of the present invention,
In the focusing in the charged particle beam apparatus based on, the maximum effective scanning interval of the charged particle beam is set in advance, and the maximum effective scanning interval is compared with the actual scanning interval based on the set magnification. If the effective scanning interval is longer than the effective scanning interval, at least the magnification in the scanning direction of the charged particle beam is increased so that the actual scanning interval is equal to or less than the maximum effective scanning interval. It does not occur and accurate autofocusing can be performed.

In the focusing in the charged particle beam apparatus according to the second aspect of the present invention, a scanning position at which the maximum value of the differential signal is obtained is detected, and an image shift is performed based on the scanning position. The scanning range of the charged particle beam at the time of raising the scanning distance is increased, so that even if the scanning interval is narrowed, the characteristic part of the sample will not be out of the scanning range of the electron beam, and accurate automatic focusing should be performed. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a sample image and a scanning spot of an electron beam when a magnification is low.

FIG. 2 is a diagram showing a relationship between a sample image and a scanning spot of an electron beam when the magnification is high.

FIG. 3 shows an example of a scanning electron microscope for carrying out the method according to the invention.

FIG. 4 is a diagram showing each signal processing waveform.

FIG. 5 is a diagram for explaining an embodiment of the present invention in which the magnification in the X direction is increased.

FIG. 6 is a diagram for explaining an embodiment of the present invention in which the magnification in the X direction and the Y direction is increased.

FIG. 7 is a diagram for explaining an embodiment of the present invention that performs image shift.

[Explanation of symbols]

 DESCRIPTION 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 Storage unit 19 Waveform change check unit 20 Image shift unit

Claims (2)

[Claims] A step of changing the intensity of a focusing lens for focusing a charged particle beam onto a sample in a step-like manner, and scanning the irradiation position of the charged particle beam on the sample each time the change occurs;
In a focusing method in which the signal obtained by this scanning is differentiated, the differentiated signal is integrated, and the focusing lens is set to the intensity at which the maximum integrated signal was obtained, the maximum of the charged particle beam is set in advance. Set the effective scanning interval, compare the maximum effective scanning interval with the actual scanning interval based on the set magnification, and if the actual scanning interval is equal to or longer than the maximum effective scanning interval, at least increase the magnification in the scanning direction of the charged particle beam. A focusing method in a charged particle beam apparatus wherein the actual scanning interval is set to be shorter than the maximum effective scanning interval. 2. The apparatus according to claim 1, wherein a scanning position at which the maximum value of the differential signal is obtained is detected, and the image shift is performed so that the scanning position falls within a scanning range of the charged particle beam when the magnification is increased. Focusing method in a charged particle beam device.