JPH1092354A - Device and method for automatic focusing of electron optics system for scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatize focusing work for an electron optics system, relieve an operator from working load and greatly improve the throughput of work in inspecting semiconductors by adding a processor for images obtained from a scanning electron microscope. SOLUTION: The refraction factor of an object lens for a scanning electron microscope 11 is changed by a focusing part 12 for focusing and the preset number of images obtained from the scanning electron microscope 11 in sequence are accumulated in an image accumulating part 13. Then, the moving range of specimen images which exist in the images is calculated by a moving range calculating part 14. A focusing control part 16 judges whether an electron optics system must be focused or not in accordance with the moving rang of the specimen images and, if it must be focused, the optics system focusing part 15 focuses the electron optics system for the scanning electron microscope 11 in accordance with the moving range of the specimen images.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic optical system automatic adjusting apparatus and an electronic optical system automatic adjusting method for automatically adjusting an electronic optical system in a scanning electron microscope.

[0002]

2. Description of the Related Art Scanning electron microscope (Scanning Electoron)
Microscope and SEM are abundantly used in a wide range of fields such as semiconductor inspection. In order to inspect a fine structure of a sample surface such as a semiconductor at a high magnification, the electron optical system of the SEM must always be accurately adjusted. That is, the electron beam emitted from the electron gun of the SEM is sufficiently narrowed down by the electron optical system, and the SEM electron optical system is adjusted so that the electron beam passes through the center of the electron optical system and is focused on one point on the sample. Otherwise, a sharp image of the microstructure of the sample cannot be obtained.

There are several types of adjustment work for such an electron optical system. Among them, adjustment work for adjusting the axis of an objective lens stop so that an electron beam passes through the center axis of the electron optical system is particularly required. Call.

If the objective lens is out of alignment, that is, if the center of the objective lens aperture is not aligned with the axis of the electron beam, when the operation of focusing on the sample is performed, the sample image is displayed in the image. Move. This situation causes so-called astigmatism in which the shape of the electron beam spot irradiated on the sample does not become a perfect circle, and lowers the resolution of the image. For this reason, usually, the operator manually operates the objective lens to alternately output an overfocus image and an underfocus image, and adjusts the position of the objective lens stop until the image does not move in the image.

[0005] The objective lens alignment is likely to be out of order due to external vibrations applied to the SEM, so it is necessary to adjust the alignment periodically or every time the SEM is used. ing. However, since it is an operation requiring skill, there are few operators who can make accurate adjustments, and it is difficult for ordinary SEM users to make accurate adjustments.

Further, since the time required for adjusting the electron optical system greatly affects the inspection throughput when the SEM is used for semiconductor inspection or the like, automation of the adjustment is desired.

[0007]

As described above, in the prior art, the adjustment of the objective lens alignment of the scanning electron microscope has been performed manually by the operator periodically or every time the operator uses the objective lens. However, this adjustment operation is a difficult operation requiring skill, and requires a long time, which causes a decrease in throughput in an inspection process such as a semiconductor inspection where SEM is frequently used.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a device for processing an image obtained from a scanning electron microscope to adjust an electron optical system. To reduce the workload of the operator and greatly improve the throughput of operations in semiconductor inspection and the like.

[0009]

SUMMARY OF THE INVENTION The present invention relates to an automatic adjustment apparatus for an electron optical system for adjusting an electron optical system of a scanning electron microscope, and focus adjustment by changing a refractive index of an objective lens of the scanning electron microscope. Focus adjusting means for performing the focusing, image accumulating means for accumulating a predetermined number of images sequentially obtained from the scanning electron microscope by the focus adjusting by the focus adjusting means, and the plurality of images accumulated in the image accumulating means. It is necessary to adjust the electron optical system of the scanning electron microscope based on the moving amount calculating means for calculating the moving amount of the existing sample image and the moving amount of the sample image calculated by the moving amount calculating means. Adjusting process control means for determining whether or not adjustment of the electron optical system of the scanning electron microscope is required by the adjusting process control means. Te is obtained by including an optical system adjusting means for adjusting the electron optical system of the scanning electron microscope.

The moving amount calculating means binarizes the plurality of images accumulated in the image accumulating means, obtains a reference position of a sample image present in the binarized image, and obtains a reference position. The amount of movement of the sample image is calculated by tracking a reference position.

Further, the moving amount calculating means calculates a threshold value for the binarizing process by performing a statistical process on the plurality of images. Further, the moving amount calculating means calculates the moving amount of the sample image by performing a correlation operation between the plurality of images stored in the image storing means.

The moving amount calculating means calculates the moving amount of the sample image by creating a focal amount space image from the plurality of images stored in the image storing means.

According to such a configuration, when a predetermined number of images are obtained from the scanning electron microscope by the focus adjustment,
The amount of movement of the sample image present in those images is determined by, for example, the position of the center of gravity of the images or the correlation calculation or the focal space image. If the moving amount of the sample image is equal to or larger than the allowable value, it is determined that the electron optical system needs to be adjusted, and the electron optical system of the scanning electron microscope is adjusted according to the moving amount.

As described above, by processing the image obtained from the scanning electron microscope, the adjustment work of the electron optical system can be automated. As a result, the workload of the operator can be reduced, and the throughput of work in semiconductor inspection and the like can be significantly improved.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 1 is a block diagram showing a configuration of an electronic optical system automatic adjusting apparatus according to a first embodiment of the present invention. This apparatus includes a scanning electron microscope 11, a focus adjustment unit 1
2, an image storage unit 13, a movement amount calculation unit 14, an optical system adjustment unit 15, and an adjustment processing control unit 16.

The scanning electron microscope 11 is used in a wide range of fields such as semiconductor inspection and the like, for inspecting a fine structure of a sample surface such as a semiconductor at a high magnification. FIG. 2 shows a schematic configuration of the scanning electron microscope 11.

The focus adjusting unit 12 includes a scanning electron microscope 11
Is adjusted by changing the refractive index of the objective lens. The image storage unit 13 captures a sample image from the detector of the scanning electron microscope 11 by focus adjustment by the focus adjustment unit 12, and stores a plurality of images. The movement amount calculation unit 14 calculates the movement amount of the sample image present in the plurality of images stored in the image storage unit 13. The optical system adjustment unit 15 adjusts (corrects) the objective lens alignment by adjusting the electron optical system of the scanning electron microscope 11. The adjustment processing control unit 16 controls the flow of the entire electron optical system adjustment processing of the scanning electron microscope 11.

FIG. 2 is a general schematic configuration diagram of the scanning electron microscope 11 in FIG. The focus adjustment unit 12 outputs a focus adjustment signal 202 under the control of the adjustment processing control unit 16,
The focus of the objective lens 21 of the scanning electron microscope 11 is adjusted. The image storage unit 13 A / D converts the image detection signal 203 output from the detector 22 of the scanning electron microscope 11 and stores a two-dimensional grayscale image. The optical system adjustment unit 15
The optical system adjustment signal 201 under the control of the adjustment processing control unit 16
Is output, and the alignment of the objective lens 21 is adjusted by adjusting the deflector 23 of the scanning electron microscope 11.

Here, in the scanning electron microscope 11,
If the alignment of the objective lens 21 is out of order,
When an operation of focusing (adjusting the refractive index of the objective lens) on the sample 24 is performed, the sample image moves in the image. Therefore, the adjustment amount of the lens alignment can be known from the moving direction and the amount of the sample image during the focus adjustment.
Then, by repeatedly adjusting the stop position of the objective lens 21 until the sample image does not move in the image, the objective lens alignment can be accurately adjusted (that is, corrected to a correct position).

In order to know the moving amount and the direction of the sample image at the time of focus adjustment, it is only necessary to take images of variously changed focuses using the same sample and measure the moving amount of the image feature with respect to the focus adjusting amount. The problem here is to realize an image processing method that can stably track an image even with respect to “blur” of a sample image due to focus adjustment.

Hereinafter, the automatic adjustment process of the electron optical system according to the first embodiment will be described. FIG. 3 is a flowchart showing the operation of the electronic optical system automatic adjustment processing in the first embodiment. A focus adjustment amount Sn to be adjusted is set in advance in the adjustment processing control unit 16 (Equation 1).

[0022]

(Equation 1)

According to the value, the focus adjusting unit 12 adjusts the focal length of the objective lens 21 of the scanning electron microscope 11 (Step A11). Thereafter, the sample 24 is photographed by the scanning electron microscope 11 and the image is stored in the image storage unit 13.
(Step A12).

The image I (n) obtained according to the focus adjustment amount Sn and stored in the image storage unit 13 is defined as x (maximum value X) in the horizontal direction of the image and y (maximum value Y) in the vertical direction. , In
It is represented as (x, y) (Equation 2).

[0025]

(Equation 2)

The stored images In (x, y) are successively sent to the movement amount calculating unit 14, and the center of gravity of each region in the image is calculated as follows. That is,
In the movement amount calculation unit 14, first, as shown in Expression 3, the image I′n after the preprocessing is performed by the preprocessing (step A13).
(X, y) is obtained.

[0027]

(Equation 3)

As the preprocessing F, for example, labeling after removing noise in the imaging system (step A1)
A noise removal process is performed to avoid overflow of the label number in 6). The simplest noise elimination processing is a simple smoothing processing by superposition calculation of a local average mask of 3 × 3 pixels as shown in FIG. In order to further improve the noise removal performance, a mask having an arbitrary size of 3 × 3 pixels or more may be superimposed, or the value of each pixel of the mask may be arbitrarily weighted. Also,
In addition to the simple smoothing process, it is also possible to remove noise using a statistical process such as a median filter.

Image I'n (x, y) subjected to preprocessing
Is subjected to a binarization process using a predetermined threshold th, and a binarized image Jn (x, y) is obtained as shown in Expression 4 (Step A14). The threshold value th may be set to one for the entire image In, or a different value may be set according to the coordinates In (x, y) of the image.

[0030]

(Equation 4) The binarized image Jn (x, y) is labeled for each connected region in the image, and a label region A represented by Expression 5
n (k) is obtained (step A15).

[0031]

(Equation 5)

Here, the maximum value of the label obtained for the image In (x, y) is Kn. The labeling process refers to a general process of performing image conversion by assigning a single label number to each independent connected image region (for example, Reference 1).

Reference 1: "Image Analysis Handbook", supervised by Takagi and Shimoda, University of Tokyo Press, ISBN 4-13-061
107-0C3050 For example, a binarized image as shown in FIG. 5 becomes as shown in FIG. 6 by the labeling process. In this figure, the maximum label number Kn is “3”, and the label area is An (1). , An
(2) and An (3). 5 and 6, (a) is a captured image of the sample, and (b) is a diagram in which the sample image is divided for each pixel. And
The x coordinate Gx (k) and the y coordinate Gy (k) of the position of the center of gravity of the image area corresponding to each label An (k) are calculated by Expression 6 (step A16).

[0034]

(Equation 6)

In practice, post-processing such as correcting the label image may be performed before calculating the center of gravity in order to calculate the center of gravity more accurately. As such post-processing, for example, a filling process of a label image, a feature amount calculating process based on a secondary moment amount or the like after the filling process, and the like are given. These processes allow only presumed image features to be extracted as label regions.

Each time the image In (x, y) corresponding to the focus adjustment amount Sn is stored in the image storage unit 13, the movement amount calculation unit 14 first performs the process of calculating the center of gravity position.
The centroid positions GXn (k) and GYn (k) of all the label areas An (k) of n (x, y) are stored. The above processing is performed by the focus adjustment amount S set by the adjustment processing control unit 16.
n (Step A)
17).

Here, for example, using a contact hole image taken in a semiconductor inspection of an LSI or the like, the contact hole image and the center of gravity position calculation when the focus adjustment amount is set in three stages of -5 μm, 0 μm, and +5 μm The results obtained are shown in FIG. In this example, the position of the center of gravity is calculated for one contact hole by the method described above.

As described above, after the images In (x, y) are photographed for all the set focus adjustment amounts Sn and the center of gravity positions GXn (k), GYn (k) are calculated,
The movement amount calculation unit 14 performs center-of-gravity tracking by associating the label regions that are the closest centers of gravity between images with different focus adjustment amounts (step A18), and calculates the image movement amount with respect to the focus adjustment amounts. (Step A19).

The center-of-gravity position tracking processing in step A18 will be described. It is assumed that Sn and Sm are a pair having close focus adjustment amounts. For example, if Sn is "-2", "-
Assuming that five levels of “1”, “0”, “+1”, and “+2” are set, “−2” and “−1”, “−1” and “0”, “0” and “+1” , “+1” and “+2”. For all combinations of the label areas An (k) and Am (k ') corresponding to the respective focus adjustment amounts Sn and Sm, the horizontal distance DXnm and the vertical distance DY
nm is calculated by equation 7.

[0040]

(Equation 7) Further, the Euclidean distance Dnm between the label areas An (k) and Am (k ′) is obtained by Expression 8.

[0041]

(Equation 8)

Then, among all combinations of An (k) and Am (k '), those having the minimum distance Dnm are regarded as corresponding label areas. The correspondence between such label areas is defined as Sn and S having the closest focus adjustment amounts.
The combination of m is performed, and all label areas obtained from all the images stored in the image storage unit 13 are associated with each other.

Next, the movement amount calculation processing in step A19 will be described. Here, An (k) and Am
(K ') is assumed to be one of the sets of label areas associated in step A18. Similarly to step A18, the horizontal and vertical distances DXnm and DYnm between Sn and Sm are obtained using Expressions 7 and 8. This distance DXn
Since m and DYnm are calculated once in a7, the values at that time may be stored in the movement amount calculation unit 14, and the values may be used. Here, as described above, Sn
Are set at equal intervals, as in the example in which five levels of -2, -1, 0, +1 and +2 are set, and all the associated label areas S
Assuming that the total number of pairs of n and Sm is Na, the image movement amounts Tx and Ty in the horizontal and vertical directions per unit focus adjustment amount are obtained using Expression 9.

[0044]

(Equation 9)

FIG. 8 shows the result of actually calculating the image movement amounts Tx and Ty. Each value in this figure shows the result of calculating the amount of movement for six contact hole images. From these results, when the focus adjustment amount is −5 μm → 0 μm, 17.12 pixels on the left (horizontal direction x), 30.74 pixels on the lower side (vertical direction y) on average, and when 0 μm → + 5 μm, the average is left. 14.15 pixels in the (horizontal direction x) and 25.4 pixels in the lower (vertical direction y)
The sample image is moving by 5 pixels, and the average image movement amount Tx
And Ty are 15.64 pixels to the left and 28.10 pixels below.

Finally, the adjustment processing control section 16 determines whether or not the electron optical system of the scanning electron microscope 11 needs to be adjusted (step A20). Here, step A19
Equation 10 determines whether or not the image movement amounts Tx and Ty calculated in are smaller than predetermined allowable movement amounts Cx and Cy. If both of the two expressions shown in Expression 10 are satisfied when the image movement amounts Tx and Ty are less than the allowable movement amounts, all the processes are completed without the necessity of adjusting the electron optical system.

[0047]

(Equation 10)

On the other hand, if it is determined in step A20 that the movement amount is equal to or larger than the allowable movement amount, then in step A19
The electron optical system of the scanning electron microscope 11 is adjusted by the optical system adjusting unit 15 in accordance with the image movement amounts Tx and Ty calculated in. Thereafter, the processing described above is repeated again.

The optical system adjusting section 15 has an image moving amount Tx,
An adjustment amount for an arbitrary lens of the scanning electron microscope 11 or the deflector 23 is set for Ty. For example, the adjustment amount Oi for a certain lens or deflector 23 is
An arbitrary function G may be set as in Expression 11, or an adjustment amount Oi may be set in a form like a table for arbitrary Tx and Ty.

[0050]

[Equation 11]

As described above, the processing shown in FIG. 3 is repeated until it is determined in step A20 that the image movement amounts Tx and Ty are less than the allowable values, whereby the electron optical system of the scanning electron microscope 11 is repeated. Can be automatically adjusted.

More specifically, as shown in FIG. 7, for example, it is assumed that three images are sequentially obtained from the scanning electron microscope 11 by three focus adjustments. The number of times of focus adjustment at this time corresponds to the number of times of imaging up to the specified focus, as shown in step A17 in FIG.

The positions of the centers of gravity of the sample images existing in these images are obtained, and the positions of the centers of gravity are tracked to obtain the amount of movement of the sample images. As described above, if the movement amount of the sample image at this time is less than the allowable value, that is, if the sample image does not move for the focus adjustment, the lens alignment is correct at that time. Therefore, the processing here is terminated as it is not necessary to adjust the electron optical system.

On the other hand, if the moving amount of the sample image is equal to or more than the allowable value, that is, if the sample image is moving with respect to the focus adjustment, the lens alignment has not yet been adjusted at that point. It is determined that the electron optical system needs to be adjusted, and after the adjustment is performed, the processing from step A1 is repeated again.

As described above, by processing the image obtained from the scanning electron microscope 11, the adjustment work of the electron optical system can be automated. In this case, as shown in FIGS. 7A and 7B, the sample image is blurred when the object is out of focus. Usually, when the sample image is blurred,
Since it is difficult to calculate the correct movement amount, it is difficult to adjust the lens alignment. However, according to the method of the present invention, the adjustment can be performed regardless of the blur of the sample image.

In the first embodiment, the tracking target is the center of gravity of the sample image. However, the tracking target may be, for example, the center position of the sample image. In short, it is only necessary that the movement amount of the sample image can be calculated by determining a reference position and tracking the position.

(Second Embodiment) Next, a second embodiment of the present invention will be described. The configuration of the device is the same as that of FIG. 1, and the description thereof is omitted here.
FIG. 9 shows a flowchart of the electronic optical system automatic adjustment processing in the second embodiment. In FIG. 9, step B1
The fourth embodiment is the same as the first embodiment except that the threshold value calculation process of No. 4 is performed in the movement amount calculation unit 14.

That is, in the first embodiment, the image I '
Although the threshold value th of the binarization process of n (x, y) (step A14 in FIG. 3) is assumed to be predetermined, even if the image is "blurred" due to the focus adjustment, the label area is appropriately adjusted. In order to extract the optimum threshold value th, a statistical process is performed in accordance with the photographed sample image.
May be more effective.

Therefore, in the second embodiment, as preprocessing of the binarization processing (step B15) for the input image,
A threshold value calculation process (step B14) is inserted. The specific processing of the threshold value calculation processing (step B14) is shown in the flowchart of FIG.

FIG. 10 is a flowchart showing the operation of the threshold value calculation processing. It is assumed that an image region Rn (x, y) for calculating a threshold value is determined for each pixel I'n (x, y) of the sample image after the preprocessing (step B13). This image area can be set arbitrarily. For example, it can be set as a rectangular area of 15 × 15 pixels centering on the pixel (x, y) or the whole image In. When the entire image is designated, the threshold value thn (x, y) obtained in the subsequent processing becomes a constant value in the image regardless of the coordinates.

First, the threshold value calculation processing (step B1)
4) In the image area Rn (x, x) of each pixel In (x, y)
The histogram of the gray value of the image is calculated in step y) (step C11). For simplicity, only the image region Rn (x, y) is considered hereafter, and the obtained histogram (frequency) at each gray value i is represented by h
i. Here, for example, when an 8-bit grayscale image is obtained from the scanning electron microscope 11, the minimum value of the grayscale value i is “1” and the maximum value imax is “255”.
It becomes a gradation image of stages.

The histogram h obtained in step C11
From i, a statistic required for the threshold determination step C13 is calculated in a statistic calculation process (step C12). Here, a threshold value calculation method based on a discriminant analysis method or a method called Otsu's method will be described.

First, the probability distribution pi for the gray value i is obtained from the histogram hi using Expression 12. Note that Nr in Equation 12 indicates the total number of pixels in the region Rn (x, y).

[0064]

(Equation 12) Next, the in-class probability sums ω0 and ω1 are obtained by Expression 13.

[0065]

(Equation 13) Then, the in-class averages μ0 and μ1 and the overall average μt are obtained by Expression 14.

[0066]

[Equation 14]

Of these, the in-class probability sums ω0, ω
1, μ0 and μ1 are obtained for all possible values of k. For each value of k, the inter-class variance E (k) is determined by equation (15).

[0068]

(Equation 15)

Finally, the threshold value determination processing (step C1)
In 3), k, which is the largest value among the inter-class variances E (k) obtained in step C12, is set to the optimal binarization threshold thn in the image region Rn (x, y).
By outputting as (x, y), an optimal binarized image Jn (x, y) is obtained by Expression 16 in the subsequent binarization processing (step B15).

[0070]

(Equation 16)

Here, the focus adjustment amount is -5 μm, 0 μm,
FIGS. 11 and 12 show examples of the binarization threshold value calculation result and the binarized image when the three levels of +5 μm are set.
As shown in FIG. In this example, the image region Rn (x, y) corresponding to each pixel (x, y) is set in the entire image In, and the threshold value thn (x, y) is a single value in one image. It has become.

Further, in the second embodiment, the method of determining the threshold value according to the Otsu method has been described. However, the present invention is not limited to this method, and the label image has a very small area with respect to the entire image. In some cases, the kittler method described below may be effective.

In the kittler method, the in-class probability sums ω0, ω
1 is obtained in the same manner as Expression 13, and when the in-class variance corresponding to the in-class probability sums ω0, ω1 is obtained as σ0, σ1, the statistic E ′ (k) obtained by Expression 17 is minimized. In this method, k is set to an optimal threshold value thn (x, y) in the threshold value determination processing step C13.

[0074]

[Equation 17]

As described above, according to the second embodiment, a threshold value for binarization processing is calculated by performing statistical processing on a plurality of images, and each image is calculated based on the threshold value. Is binarized. As a result, even when the image is "blurred" due to the focus adjustment, the label area can be properly extracted, and more accurate optical system adjustment can be realized.

(Third Embodiment) Next, a third embodiment of the present invention will be described. The configuration of the device is the same as that of FIG. 1, and the description thereof is omitted here.
FIG. 14 shows a flowchart of the electronic optical system automatic adjustment processing in the third embodiment. In FIG. 14, focus adjustment (step D11), image input (step D12), pre-processing (step D13), determination of photographing end (step D)
14), the determination of the need for optical system adjustment (step D17), and the adjustment of optical system (step D18) are the same as the corresponding processes in FIG. 3 described in the first embodiment.
Here, the description is omitted.

In the third embodiment, there is no processing for obtaining a binarized image or calculating the position of the center of gravity after the preprocessing (step D13) as in the first embodiment. There is a process of calculating the moving amount of the image (step D16) and a process of calculating the correlation value between the image regions (step D15), and these processes are performed by a method unique to the third embodiment.

After the processing up to step D14 has been completed, a prescribed number of images I'n
It is assumed that (x, y) is accumulated. Further, as in the first embodiment, it is assumed that the focus adjustment amounts Sn are set at equal intervals, and that Sn and Sm are a pair of the closest focus adjustment amounts.

In the third embodiment, the focus adjustment amount Sn
And the image movement amounts Tx and Ty are calculated such that the correlation value between the images I'n and I'm obtained corresponding to Sm and Sm becomes the highest. (Step D18).

FIG. 15 shows a preprocessed image I'n obtained from the sample image In obtained in accordance with the focus adjustment amount Sn.
It is assumed that an image area Rn and a search area Un are set as shown in FIG.

In FIG. 15, the image area Rn and the search area Un are drawn by respective rectangular areas. However, they can be set in any shape and at any position.
Both can also be set for the entire image.

In the correlation value calculation process (step D15),
Between an image region Rn set by the sample image I'n with the focus adjustment amount Sn and an arbitrary image region Rm included in the search region Um set by the image I'm with the focus adjustment amount Sm Then, a statistic indicating the degree of correlation between the respective regions is calculated. For example, the reference coordinates of the image area Rn are (x1, y
1) With the reference coordinates of the image region Rm being (x2, y2), the residual θ is calculated using Expression 18.

[0083]

(Equation 18)

The method using such a residual θ is based on SSD
There is also a method with a small calculation cost such as the A method, and it is a simple method for obtaining the degree of correlation. Also, Rn and Rm
May be obtained. Such a correlation value calculation is performed between one Rn and all Rm included in Um.

Next, the movement amount calculation processing (step D16)
In the case where the residual θ is obtained, for example, R ′ is set such that the residual θ is the smallest with respect to the image area Rn.
m, and the difference between the reference coordinates (x1, y1) and (x'2, y'2) of each of the image regions Rn and R'm is calculated using Equation 19, and the image is moved between the focus adjustment amounts Sn and Sm. It can be calculated as the quantities Tx, Ty. The image movement amounts Tx and Ty are calculated for all sets of the focus adjustment amounts Sn and Sm, and the total average value of Tx and Ty obtained in each set is output as the image movement amount.

[0086]

[Equation 19]

The image area Rn and the search area Un may be set not only for one image I'n but also for a plurality of them. In this case, the total average value of all Tx and Ty is shifted. The amount is output as the image movement amount from the amount calculation unit 14, and can be used as an index for the optical system adjustment by the optical system adjustment unit 15 (step D18).

As described above, according to the third embodiment, the amount of movement of the sample image is obtained by performing a correlation operation between a plurality of images. The moving amount of the sample image can be easily obtained without the need for the tracking process.

(Fourth Embodiment) Next, a fourth embodiment of the present invention will be described. The configuration of the device is the same as that of FIG. 1, and the description thereof is omitted here.
FIG. 16 shows a flowchart of the electronic optical system automatic adjustment processing in the fourth embodiment. In FIG. 16, focus adjustment (step E11), image input (step E12), pre-processing (step E13), determination of photographing end (step E)
14), the determination of the necessity of the optical system adjustment (step E17), and the optical system adjustment (step E18) are the same as the corresponding processes in FIG. 3 described in the first embodiment.
Here, the description is omitted.

In the fourth embodiment, there is no processing for obtaining a binarized image or calculating the position of the center of gravity after the preprocessing (step E13) as in the first embodiment. There is a process of calculating the moving amount of the image (step E16) and a process of creating a focal amount space image (step E15) therefor, and these processes are performed by a method unique to the fourth embodiment.

After the processing up to step E14 has been completed, the prescribed number of images I'n
It is assumed that (x, y) is accumulated. It is assumed that a large number of focus adjustment amounts Sn are set at equal intervals. At this time, an image portion having a specific y-coordinate value from each image I′n (x, y), that is, a slit image of one horizontal line corresponding to a certain y-coordinate value is vertically arranged in the order of n. 17, an image in which the horizontal axis is the x coordinate and the vertical axis is the focus adjustment amount Sn can be created.

Such an image is herein referred to as a focal amount space image. Further, by arranging the slit images of one vertical line corresponding to a certain x coordinate value in the vertical direction in the order of n, it is possible to create a focal amount space image for y. In this manner, in the focal point space image creation processing (step E15), two focal point space images for the preset x and y coordinate values are created.

From the focal space image created in this way, the movement amounts Tx and Ty of the sample image with respect to the focus adjustment amount are calculated as follows (step E16). First,
By subjecting the two focus amount space images to spatial differentiation processing such as a sobel operator, only straight edges generated by focus adjustment are extracted as shown in FIG. And
By processing such as Hough transform for obtaining the inclination of a straight line portion in an image (Reference Document 1), all the inclinations bi of a plurality of straight lines included in each focal amount space image are obtained, and the total average value b of bi is obtained. .

The average inclination b (the average value of the inclination of the straight line portion in the image of FIG. 18) indicates the amount of movement of the sample image in the x direction or y direction with respect to the unit focus adjustment amount.
Accordingly, the average tilt b obtained from the focal amount space image in the x direction is output from the moving amount calculating unit 14 as Tx, and the average tilt b obtained from the focal amount space image in the y direction is output as Ty from the moving amount calculation unit 14. It can be used as an index for adjusting the optical system in the unit 15 (step E1).
8).

As described above, according to the fourth embodiment, a focal space image is created from a plurality of images, and the amount of movement of the sample image is obtained from the focal space image. The amount of movement of the sample image can be obtained more easily without the need for calculating and tracking the center of gravity position.

The method described in each of the above-described embodiments is a program that can be executed by a computer, such as a magnetic disk (floppy disk, hard disk, etc.), an optical disk (CD-ROM, It is also possible to write the data on a recording medium such as a DVD or a semiconductor memory and apply it to various devices, or to transmit it via a communication medium and apply it to various devices.
Also, the computer that realizes the present apparatus reads the program recorded on the recording medium, and executes the above-described processing by controlling the operation of the program.

[0097]

As described above, according to the present invention, the adjustment work of the electron optical system can be automated by adding a device for processing an image obtained from a scanning electron microscope. Therefore, conventionally, the adjustment of the electron optical system of the scanning electron microscope is a difficult operation requiring skill and cannot be adjusted by ordinary users, or the adjustment of the electron optical system in the inspection process such as semiconductor inspection. Although there was a problem that the inspection throughput was lowered due to this, according to the present invention, it is possible to reduce the work load of the operator and greatly improve the throughput of the operation in the semiconductor inspection and the like. Is played.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an electronic optical system automatic adjusting device of the present invention.

FIG. 2 is a diagram showing a schematic configuration of a scanning electron microscope applied to the electron optical system automatic adjusting device.

FIG. 3 is a flowchart illustrating an operation of an electronic optical system automatic adjustment process according to the first embodiment of the present invention.

FIG. 4 is a view showing a 3 × 3 pixel mask for simple smoothing processing according to the first embodiment;

FIG. 5 is a diagram illustrating an example of a binarized image according to the first embodiment.

FIG. 6 is a diagram illustrating an example of a label image according to the first embodiment.

FIG. 7 is a diagram illustrating an example of calculating a center of gravity position in focus adjustment according to the first embodiment.

FIG. 8 is a diagram illustrating an example of calculating a moving amount of a sample image with respect to focus adjustment according to the first embodiment.

FIG. 9 is a flowchart illustrating an operation of an electronic optical system automatic adjustment process according to the second embodiment of the present invention.

FIG. 10 is a flowchart illustrating an operation of a threshold value calculation process according to the second embodiment.

FIG. 11 is a diagram illustrating an example of a threshold value calculation result and a binarized image at a focus adjustment of −5 μm according to the second embodiment.

FIG. 12 is a diagram illustrating an example of a threshold value calculation result and a binarized image at a focus adjustment of 0 μm according to the second embodiment.

FIG. 13 is a diagram illustrating an example of a threshold value calculation result and a binarized image at a focus adjustment of +5 μm according to the second embodiment.

FIG. 14 is a flowchart illustrating an operation of an electronic optical system automatic adjustment process according to the third embodiment of the present invention.

FIG. 15 is a diagram illustrating a correlation value calculation area according to the third embodiment.

FIG. 16 is a flowchart illustrating an operation of an electronic optical system automatic adjustment process according to the fourth embodiment of the present invention.

FIG. 17 is a diagram illustrating an example of a focus amount space image according to the fourth embodiment.

FIG. 18 is a diagram showing an example of straight line extraction of a focal amount space image according to the fourth embodiment.

FIG. 19 is a view showing a method of supplying the apparatus by software.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Scanning electron microscope 12 ... Focus adjustment part 13 ... Image accumulation part 14 ... Movement amount calculation part 15 ... Optical system adjustment part 16 ... Adjustment processing control part 21 ... Objective lens 22 ... Detector 23 ... Deflector 24 ... Sample 201 ... Optical system adjustment signal 202 ... Focus adjustment signal 203 ... Image detection signal

Claims (10)

[Claims] 1. An electron optical system automatic adjusting apparatus for adjusting an electron optical system of a scanning electron microscope, comprising: a focus adjusting means for adjusting a focus by changing a refractive index of an objective lens of the scanning electron microscope; An image storage means for storing a predetermined number of images sequentially obtained from the scanning electron microscope by the focus adjustment by the focus adjustment means, and a moving amount of a sample image present in the plurality of images stored in the image storage means. Moving amount calculating means for calculating; and adjusting processing for determining whether or not adjustment of the electron optical system of the scanning electron microscope is necessary based on the moving amount of the sample image calculated by the moving amount calculating means. Control means; and if the adjustment processing control means determines that the electron optical system of the scanning electron microscope needs to be adjusted, the scanning electron microscope is adjusted according to the amount of movement of the sample image. An electron optical system an automatic adjustment device, characterized by comprising an optical system adjusting means for adjusting the electron optical system. 2. The method according to claim 1, wherein the moving amount calculating unit performs a binarization process on the plurality of images stored in the image storing unit, and obtains a reference position of a sample image present in the binarized image. 2. The electron-optical-system automatic adjusting apparatus according to claim 1, wherein the movement amount of the sample image is calculated by tracking the reference position. 3. The electro-optical device according to claim 2, wherein the movement amount calculation means calculates a threshold value for the binarization process by performing a statistical process on the plurality of images. Automatic adjustment system. 4. The apparatus according to claim 1, wherein the moving amount calculating means calculates the moving amount of the sample image by performing a correlation operation between the plurality of images stored in the image storing means. Electronic optical system automatic adjustment device. 5. The moving amount calculating unit calculates a moving amount of the sample image by creating a focal amount space image from the plurality of images stored in the image storing unit. 2. The electronic optical system automatic adjusting device according to 1. 6. A method of automatically adjusting an electron optical system of an electron optical system for adjusting an electron optical system of a scanning electron microscope, the method comprising: adjusting a focus of the scanning electron microscope by changing a refractive index of an objective lens of the scanning electron microscope. Then, a predetermined number of images sequentially obtained from the scanning electron microscope are accumulated by the focus adjustment, a moving amount of a sample image present in the accumulated plurality of images is calculated, and the calculated sample is calculated. Based on the amount of movement of the image, it is determined whether or not the electron optical system of the scanning electron microscope needs to be adjusted.If it is determined that the electron optical system of the scanning electron microscope needs to be adjusted. Wherein the electron optical system of the scanning electron microscope is adjusted according to the amount of movement of the sample image. 7. After the plurality of images are binarized, a reference position of a sample image present in the binarized image is obtained, and the reference position is tracked to thereby move the sample image. 7. The method for automatically adjusting an electron optical system according to claim 6, wherein 8. The method according to claim 7, wherein a threshold value for the binarization process is calculated by performing a statistical process on the plurality of images. 9. The method according to claim 6, wherein the amount of movement of the sample image is calculated by performing a correlation operation between the plurality of images. 10. The method according to claim 6, wherein the movement amount of the sample image is calculated by creating a focal amount space image from the plurality of images.