JP2000182555A - Automatic focusing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve accuracy of automatic focusing of an automatic focusing device by reducing focusing time. SOLUTION: This automatic focusing device 1 is equipped with evaluation means 11, 12 with different detection accuracy of focal position, and focuses by using these evaluation means. The evaluation means 11 with coarse detection accuracy of the focal position focuses roughly in a short period by using evaluation values obtained by broad detection of the focal position. The evaluation means 12 with fine detection accuracy of the focal position focuses very precisely by using evaluation values obtained by narrow detection in vicinity of the focal point obtained by the coarse detection means 11. With a combination of different evaluation values of different characteristics, a period of focusing is reduced and accuracy of focusing is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for focusing a beam in an apparatus using a charged particle beam, such as an electron microscope, a conductor manufacturing apparatus, a precision machine, and a biotechnology field.

[0002]

2. Description of the Related Art In an electron microscope, a semiconductor manufacturing apparatus, or the like, a sample is analyzed, observed, or processed by irradiating the sample with a charged particle beam such as an electron beam or an ion beam. In such an apparatus using a charged particle beam, it is necessary that the beam be focused on the sample in order to increase the analysis accuracy and processing accuracy.

Conventionally, beam focusing is performed by a measurer who judges a measurement result or obtains an evaluation value for evaluating an image with respect to image data obtained by an electron microscope, and uses this evaluation value to adjust the beam focus. The measurement is performed by adjusting measurement conditions such as the power of the objective lens so as to match.

[0004]

Generally, as characteristics for evaluating focusing itself by image evaluation, both characteristics of focusing accuracy and time required for focusing are considered.
Normally, it is difficult to achieve both characteristics at the same time. When focusing is performed in a short period of time, the accuracy of focusing is low, and when performing high-precision focusing, it takes a long time to perform focusing. There is a problem. In particular, in the case of high-precision focusing, if the change width of the focus is made large and the change is made coarsely, it becomes difficult to perform focusing if the focus is located within this change width. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems, to provide an automatic focusing device capable of shortening the time required for focusing and improving the accuracy of focusing.

[0005]

SUMMARY OF THE INVENTION The present invention relates to an automatic focusing apparatus for performing focusing by changing focusing parameters using an evaluation value obtained based on image data, wherein the evaluation values have different characteristics. By combining the above, the time required for focusing can be reduced and the accuracy of focusing can be increased.

The automatic focusing apparatus of the present invention includes a plurality of evaluation means having different detection accuracy of the focus position, and performs focusing by combining these evaluation means. The evaluation means having a coarse focus position detection accuracy retrieves the focus position in a wide range to obtain an evaluation value. Using this evaluation value, rough focusing is performed in a short time.

[0007] Next, the evaluation means having a fine focus position detection accuracy retrieves the vicinity of the focus position obtained by the coarse evaluation means in a narrow range to obtain an evaluation value. Focusing is performed with high accuracy using this evaluation value. Normally, high-precision focusing takes a long time, but the fine evaluation means searches in a narrow range near the focal position obtained by the coarse evaluation means, so that the focusing time can be reduced.

The evaluation value is obtained by digitizing a signal change obtained by scanning a beam with a certain focusing parameter on one screen or one line, and evaluating the degree of focusing based on the numerical value. The power supplied to the objective lens can be used as a focusing parameter, and the focusing can be performed by adjusting the power of the objective lens based on the evaluation value.

In the case where an image obtained by beam scanning is divided into a plurality of regions, one aspect of the evaluation means is that the signal intensity on one screen or one line detected by beam scanning has a maximum signal intensity and a minimum signal intensity. Can be obtained as an evaluation value. The evaluation value obtained by this evaluation means is in a simple increasing relationship with the degree of focusing, and the greater the difference between the maximum signal intensity and the minimum signal intensity, the better the focusing. The calculation for obtaining this evaluation value is 1
On the screen or on one line, the maximum signal intensity and the minimum signal intensity are simply obtained and the difference between them is calculated, so that it can be performed in a short time, but the focus position detection accuracy is not high. Therefore, this evaluation means is suitable for the evaluation means of the present invention having a coarse focus position detection accuracy.

In another aspect of the evaluation means, a difference in signal intensity between adjacent regions is obtained for a signal intensity on each minute region detected by beam scanning, and the absolute value of the difference is set to 1
A difference integrated value obtained by adding on the screen or one line, or a difference average value of the average of the difference integrated values can be obtained as the evaluation value. The evaluation value obtained by this evaluation means is in a relationship between the degree of focusing and the simple increase, and the larger the value, the better the focusing.
This calculation for obtaining the evaluation value requires the calculation time because the absolute value of the signal intensity difference between the adjacent minute regions is added on one screen or one line in the entire region. Is expensive. Therefore, this evaluator is suitable for the estimator of the present invention that has a high focus position detection accuracy.

The minute area for detecting the intensity of the image obtained by the beam scanning can correspond to each pixel for detecting the image, and the evaluation means evaluates using the signal intensity of the detection signal obtained from each pixel. Value can be obtained.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic block diagram of a configuration example of the automatic focusing apparatus of the present invention. In FIG. 1, a measurement device 2 is a measurement portion provided in an electron microscope or a semiconductor manufacturing device, and detects a beam irradiation state on a sample as image data. The detected image data is transmitted to the automatic focusing device 1 via the control device 3. Further, the measuring device 2 receives a focus control signal from the automatic focusing device 1 and performs focusing by controlling focusing parameters such as the power of the objective lens.

The automatic focusing apparatus 1 includes a first evaluating means 11 and a second evaluating means 12 for obtaining an evaluation value used for focusing, an image memory 13 for storing image data obtained by the measuring apparatus 2, An operation memory 14 for temporarily storing data used in the operation performed by the first evaluation unit 11 and the second evaluation unit 12 is provided.

The outline of the operation of automatic focusing according to the present invention will be described with reference to the flowchart of FIG. 2 and the graph of FIG. The automatic focusing device 1 performs coarse focus position detection using the first evaluation value obtained by the first evaluation means. In this coarse focus position detection, coarse focus adjustment is performed in a short time by searching for the focus position in a wide range and obtaining an evaluation value with a large change width.

FIG. 3A shows a first example of a change in the focal position.
Shows the change in the evaluation value. In FIG. 3, W is a value corresponding to the focal position. The first evaluation means sets a wide first focus position range as a search range, acquires image data for each focus position having a large variation width D within this range,
A first evaluation value is obtained based on the acquired image data.
Here, the first evaluation unit employs an evaluation unit suitable for focusing performed by acquiring image data with a large change width in a wide search range. In FIG. 3A, it is shown that the larger the value of the first evaluation value, the better the focusing, and the first evaluating means performs the focusing at W1.

Since the resolution of the focus position by the first evaluation means is determined by the change width D, the obtained focus position W1 is a coarse focus position (step S1). Next, after the focus position is roughly determined in step S1, near the focus position, fine focus position detection is performed using the second evaluation value obtained by the second evaluation means. In this precise focus position detection, focusing is performed with high accuracy by searching for the focus position in a narrow range and obtaining an evaluation value with a small change width.

FIG. 3 (b) shows a second example with respect to a change in the focal position.
Shows the change in the evaluation value. The second evaluation means sets the vicinity from the focus position W1 obtained by the first evaluation means as a second focus position range and a search range. The second focus position range can be, for example, a range within the change width D around the focus position W1.

The second evaluation means sets a narrow second focus position range as a search range, and a small change width d within this range.
The image data is acquired for each focal position of and the second evaluation value is obtained based on the acquired image data. Here, the second evaluation unit employs an evaluation unit suitable for focusing performed by acquiring image data with a small change width in a narrow search range. FIG. 3B shows that the larger the value of the second evaluation value is, the better the focusing is.
According to the evaluation means, focusing is performed at W2.

Since the resolution of the focal position determined by the second evaluation means is determined by the change width d, the focal position W2 to be determined can be precisely determined (step S2). next,
A configuration example of the first and second evaluation units of the automatic focusing apparatus of the present invention will be described. The first and second evaluation means obtain an evaluation value based on the image data obtained by the beam scanning, and perform focusing based on the evaluation value. First, FIGS.
The acquisition of image data by beam scanning will be described below, and focusing using image data will be described with reference to FIGS.

FIGS. 4 and 5 are diagrams for explaining image data evaluated by the first and second evaluation means. FIG. 4 shows an area for acquiring image data, and FIG. Shows the relationship. In FIG. 4, reference numeral 23 indicates a beam scanning range in the measuring apparatus. The beam emitted from the beam source is swung in a two-dimensional direction by a scanning lens or the like, and scans the scanning range 23. The measuring device acquires image data obtained by beam scanning and performs focusing. The image data can be obtained by dividing the scanning range 23 into a plurality of minute regions 20 and by using each minute region 20 as a unit. The minute area 20 can correspond to, for example, a pixel of the detection element. As for the correspondence between the minute region 20 and the pixel, one pixel can correspond to one minute region 20 or a plurality of pixels can correspond to one minute region 20.

FIG. 4 shows an example of a configuration in which the scanning range 23 is divided into n × n minute areas 20. Each minute area 20
Include (1,1), (1,2), (1,3), ...
(1, n-1), (1, n), ... (n, n-1),
(N, n), and the signal intensities detected in each minute area 20 are represented by I1,1, I1,2, I1,3,.
1, In, n-1 and In, n. The first and second evaluation means provide signal intensities I1,1 to In, detected in each minute area 20.
Focusing is performed by obtaining a first evaluation value and a second evaluation value based on n-1 and In, n.

FIGS. 5 (a) and 5 (b) and FIGS. 5 (c) and 5 (d)
3 shows a scanning state at a different focus position. FIG.
5A, the beam 21 whose focal position is W irradiates the irradiation region 24a on the scanning region 23, and the beam 21 whose focal position is W irradiates the irradiation region 24b on the scanning region 23 in FIG. The focal position W represents the distance between the surface of the irradiation area 24a shown in FIG. 5A and the focal point of the beam, and can be represented by the power supplied to the objective lens 22 that narrows the beam. As the focal position W increases, the objective lens power decreases. Conversely, as the focal position W decreases, the objective lens power increases. Therefore, focusing is performed by adjusting the objective lens power based on the first evaluation value and the second evaluation value.

FIGS. 5A and 5B show the case where the focal position W is large, and FIGS. 5C and 5D show the case where the focal position W is small. , The signal intensity is detected in each minute area (pixel) 20. When the focal position W is large, the irradiation area 24a is widened, so that beam detection is performed in a large number of minute areas (pixels) 20, but the signal intensity is low. On the other hand, when the focal position W is small, the irradiation area 24b
, The beam detection with a high signal intensity is performed in a small number of small regions (pixels) 20.

Next, FIG. 6 is a flowchart, and FIG.
FIG. 8 illustrates the evaluation value, FIG. 8 illustrates the second evaluation value, and FIG. 9 illustrates the relationship between the first evaluation value and the second evaluation value. Will be described. In the flowchart of FIG.
Steps S10 to S16 show the operation by the first evaluation unit shown in step S1 of the flowchart of FIG. 2, and steps S12 to S27 show the operation by the second evaluation unit shown in step S2 of the flowchart of FIG. Is shown.

First, the first focus position range searched by the first evaluation means is defined as a minimum focus position WA and a maximum focus position WB.
, And a focus change width D to be changed within the first focus position range and a focus change width d to be changed within the second focus position range are set. The change width d
Is a change width smaller than the change width D. These first focus position range, second focus position range, change width D, and change width d
Have the relationship shown in FIG. 9 and correspond to the respective focus position ranges and the change widths in FIG. 3 (step S10).

The control device 3 adjusts the power of the objective lens based on the command from the first evaluating means 11 and sets the focal position W to WA, which is the end of the first focal position range (step S11). The control device 3 scans the beam at the focal position WA to acquire image data and stores it in the image memory 13 (step S12).

After acquiring the image data at the focal position WA, the focal position W is changed by the change width D (W + D).
(Step S13), the beam is scanned at this focal position (W + D) to acquire image data, and stored in the image memory 13 (Step S12). While changing the focal position W by the change width D, the acquisition of image data is repeated until the focal position W becomes the maximum focal position WB (step S1).
4).

By the steps S12 to S14, it is possible to acquire and store the image data at the focal position for each width D in the first focal position range.

Next, a first evaluation value is obtained from the image data obtained in steps S15 and S16. The first evaluation means 11 reads out the image data acquired at each focal position W from the image memory 13, converts the signal intensity I into a numerical value, and
The maximum value and the minimum value are obtained on the screen or on one line, and the absolute value | maximum value−minimum value | of the difference is obtained and set as the first evaluation value S1 (step S15).

The first evaluation value S1 is an index indicating the degree of focusing. The first evaluation value S1 has a small value when the focus position W is large, and has a large value when the focus is good and the focus position W is small. When the focal position W is large as shown in FIGS. 5A and 5B, the number of pixels for which the medium signal intensity I is detected occupies a large number, and the signal intensity I and the number of pixels in FIG. Is a distribution indicated by a broken line in the relationship diagram of FIG. on the other hand,
When the focal position W is small as shown in FIGS. 5C and 5D, the distribution includes the pixels from the large signal intensity I to the small pixel, and has the distribution indicated by the solid line in the relationship diagram of FIG.

In this distribution, when the difference between the maximum value and the minimum value of the signal intensity I is obtained, the first evaluation value when the focal position W is large is S1a, and the first evaluation value when the focal position W is small. The value is S1b, and the degree of focusing can be evaluated by the value of the first evaluation value S1. In the distribution diagram of FIG. 7, the end of each distribution curve is easily affected by noise in image detection. The effects of this noise can be eliminated by averaging the signal strength at the end of the distribution curve.

Each of the first evaluation values S obtained in step S15
The focal point W at this time is determined as the focal point W1 by the first evaluation means. FIG.
W1 in (a) indicates the focal position W1 obtained by the first evaluation means (step S16).

Next, the focal position W1 obtained by the first evaluation means
Is performed with higher accuracy in the vicinity of. The control device 3 adjusts the power of the objective lens based on the instruction from the second evaluating means 12 based on the focal position W1, and sets the focal position W to W1-kD, which is the end of the second focal position range. Note that k is a coefficient between 0 and 1 and W1-kD, so that the focal position within the change width D from the focal position W1 is set as the end of the second focal position range (step S2).
1). The controller 3 scans the beam at the focal position W1-kD (W) to acquire image data, and acquires the image data.
(Step S22).

After obtaining the image data based on the focal position W1-kD (W), the focal position W is changed by the change width d to W1-kD + d (W + d) (step S23).
The beam is scanned at this focal position W1-kD + d (W + d) to acquire image data, and stores it in the image memory 13 (step S22). While changing the focal position W by the change width d, the acquisition of image data is repeated until the focal position W becomes W1 + kD, which is the end of the second focal position range (step S24).

By the steps S22 to S24, it is possible to acquire and store the image data at the focal positions for each width d in the second focal position range.

Next, a second evaluation value is obtained from the image data obtained in steps S25 and S26. The second evaluation means 12 reads out the image data obtained at each focal position W from the image memory 13, quantifies the signal intensity I, obtains the difference in signal intensity between adjacent pixels, and displays this absolute value on one screen. Alternatively, a difference integrated value obtained by integrating on one line, or a difference average value obtained by averaging the difference integrated values is obtained and set as a second evaluation value S2.

When the difference in signal strength between pixels adjacent in the line direction is used, the second evaluation value S2 is expressed by the following equation (step S25). S2 = | I1,2-I1,1 | + | I1,3-I1,2 | + | I1,4-I1,3 | +... + | In, n-In, n-1 | = Σ | (In, n−In, n−1) | The second evaluation value S2 is an index indicating the degree of focusing. When the focus position W is large, the value is small. If the value is small, the value is large. When the focal position W is large as shown in FIGS. 5A and 5B, there is no large difference between the signal intensities I between adjacent pixels, and the difference between the signal intensity I and the number in FIG. In the relationship diagram, the distribution is indicated by a broken line. On the other hand, FIG.
As shown in (d), when the focal position W is small, there is a case where the difference between the signal intensities I between adjacent pixels is large, and the distribution shown by the solid line in the relationship diagram of FIG.

In this distribution, when the absolute value of the difference between the signal intensities I is integrated, the second evaluation value when the focal position W is large is S2a, and the second evaluation value when the focal position W is small is S2b. Become. Since the absolute value of the difference between the signal intensities I included in S2b is larger than the absolute value of the difference between the signal intensities I included in S2a, the sum S2b is larger than S2a, and the value of the second evaluation value S2 The degree of focusing can be evaluated.

Each second evaluation value S obtained in step S25
The focal position W at this time is determined as the focal position W2 by the second evaluation means. FIG.
W2 in (b) indicates the focal position W2 obtained by the second evaluation means (step S26).

The control device 3 inputs the focal position W2 obtained in step S26 from the second evaluation means 12 and performs focusing (step S27). In obtaining pixel data, the first evaluation unit obtains the first image data.
Assuming that the focal position range is 4 mm and the change width D is 0.05 mm, the focal position W1 having a resolution of 0.05 mm can be obtained. Further, in the image data acquisition performed by the second evaluation means, the second focus position range is set to ± 25 μm, and the change width d
Is 1 μm, a focal position W2 having a resolution of 1 μm
Can be requested.

At this time, the first evaluator acquires image data for 80 screens, and the second evaluator acquires image data for 50 screens. If image data is acquired in the first focal position range of 4 mm with a change width d1 μm, it is necessary to acquire image data for 4000 screens, which requires a great amount of measurement time. The evaluation means is not limited to the above example, and may be another evaluation means. Further, the number of the evaluation means having different detection accuracy of the focus position is not limited to two, and a plurality of evaluation means can be used.

[0042]

As described above, according to the present invention,
The time required for focusing can be reduced, and the accuracy of focusing can be increased.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a configuration example of an automatic focusing apparatus according to the present invention.

FIG. 2 is a flowchart illustrating a schematic operation of an automatic focusing performed by the automatic focusing apparatus of the present invention.

FIG. 3 is a graph for explaining a schematic operation of an automatic focusing performed by the automatic focusing device of the present invention.

FIG. 4 is a diagram showing an area where image data to be evaluated by first and second evaluation means of the present invention is obtained.

FIG. 5 is a diagram showing a relationship between image data evaluated by first and second evaluation means of the present invention and a focus;

FIG. 6 is a flowchart illustrating automatic focusing using image data according to the present invention.

FIG. 7 is a diagram illustrating a first evaluation value of automatic focusing using image data according to the present invention.

FIG. 8 is a diagram illustrating a second evaluation value of automatic focusing based on image data according to the present invention.

FIG. 9 is a diagram illustrating a relationship between a first evaluation value and a second evaluation value according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Automatic focusing device, 2 ... Measuring device, 3 ... Control device, 11 ... 1st evaluation means, 12 ... 2nd evaluation means, 13 ...
Image memory, 14 ... operation memory, 20 ... pixels, 21 ...
Electron beam, 22 ... objective lens, 23 ... scanning area, 24
a, 24b: irradiation area, D, d: change width, I: signal intensity, S: evaluation value, S1: first evaluation value, S2: second evaluation value, W: focal position.

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Claims (1)

[Claims] 1. An automatic focusing apparatus for performing focusing by changing a focusing parameter using an evaluation value obtained based on image data, wherein a plurality of evaluation means having different focus position detection accuracy are provided. The evaluation means having a coarse focus position detection accuracy obtains an evaluation value over a wide range to perform coarse focusing, and the fine focus position detection accuracy has a fine focus position obtained by the coarse evaluation means. Automatic focusing device that obtains an evaluation value in a narrow range in the vicinity of and performs focusing.