JP2511903B2 - Image correction device for scanning electron microscope - Google Patents

Description

The present invention relates to a scanning electron microscope, and more particularly, to image distortion caused by a magnetic field of a power source from a switchboard or a cable, or image distortion due to scanning vibration during repeated image observation. To correct
The present invention relates to an image correction apparatus for a scanning electron microscope, which is suitable for obtaining a scanned image with less distortion of the shape of a pattern on the image and a length error.

[Conventional technology]

As described in Japanese Patent Application No. 61-125731 (filed on June 2, 1986) filed by the present applicant, the conventional technique is to correlate between image lines from the corrugated vibration distortion appearing on the scanned image. A method has been adopted in which the scanning vibration is estimated based on the information and the vibration distortion is corrected. However, when the factor of the image vibration distortion is the power supply magnetic field that has passed through the magnetic shield, the distortion due to the vibration of the electron beam changes slowly on the image. This is because the start of electron beam scanning is usually synchronized with the power supply phase to avoid random vibration distortion, and the distortion between scanning lines is almost equal. When the shape distortion on the image changes slowly, the image is not locally significantly deformed, so it is considered that the vibration distortion cannot be estimated from the correlation information between the image lines. Didn't.

In addition, Japanese Patent Application No. 60-228683 (filed on October 16, 1985)
Then, the displacement between the frame images during repeated image observation was detected by calculating the two-dimensional cross-correlation between the small regions selected in the frame images.

[Problems to be solved by the invention]

The above-mentioned prior art does not consider that the correlation information between the image lines becomes equal to that when there is no distortion when the vibration distortion changes slowly on the image. However, there is a problem in that it is impossible to estimate and correct distortion.

Further, in the above-mentioned conventional technology, if the S / N of the frame image is good, or if the S / N is a little bad, but the observation target has a sufficient texture, there is no problem in the positional deviation detection accuracy. In the case of an observation target such as a semiconductor with a poor / N and a large number of flat portions, erroneous detection due to noise occurred. or,
On the other hand, a huge amount of calculation was required to maintain sufficient detection accuracy regardless of the observation target.

A first object of the present invention is to provide an image correction apparatus for a scanning electron microscope capable of estimating and correcting image distortion, which slowly changes on an image due to a power source magnetic field, by image processing.

A second object of the present invention is to detect a positional shift between frame images with a small amount of calculation and in a case where the projection data in a certain direction among the low S / N frame images shows a pattern peculiar to the image. An object of the present invention is to provide an image correction apparatus for a scanning electron microscope, which is capable of performing with high accuracy and removing image vibration during repeated observation.

[Means for solving problems]

The first purpose is to obtain scan image data under a certain sync phase of a power source and store it in an image memory, then slightly change the sync phase and re-input the image data of the same field of view as a distortion change image, The difference between both image data is calculated by a calculator, and the differential value of the image data in the scanning direction and the vertical scanning direction is calculated by a differential calculator. Based on the least squares method, the power source magnetic field distortion intensity on the image is estimated, and the estimated distortion is achieved by correcting the image data on the image memory.

The second object is achieved by obtaining the projection data in the direction in which the projection data shows a unique pattern for each frame and performing the one-dimensional cross-correlation calculation using the projection data.

[Action]

FIG. 2 shows the principle of image distortion estimation according to the present invention. FIG. 2 (a) shows a coordinate system of an electron beam scan image output from the scanning electron microscope. The figure (b) shows the coordinate system placed on the imaging target. The electron beam is not disturbed,
When scanning the imaging target correctly, output image coordinates (s,
t) corresponds to the imaging target coordinate (x, y). But now 50
The power supply stray magnetic field of Hz causes deviation in the electron beam scanning, and the scanning period is 40 ms or several times that in the slow scan mode of a normal scanning electron microscope, and the synchronization with the power supply phase When the phase lock in which the electron beam scanning is started at the time of is θ, the output image is distorted as follows. The state of distortion is schematically shown in FIG.

The point (x, y) on the imaging target coordinate corresponding to the output image coordinate (s, t) is expressed by the following equation.

x = s + αsin (ωs + θ) (1) y = t + βsin (ωs + θ) (2) where α and β are the amplitudes of electron beam deviations (vibrations due to a stray magnetic field) in the scanning direction and the scanning vertical direction, respectively, and ω is the vibration. The period, the vibration period and the phase are known in the equations (1) and (2), and the vibrations α and β are the vibration parameters to be estimated.

The image intensity f (x, y) at the imaging target coordinates (x, y) is
The output image intensity on the output image coordinates (s, t) is geometrically distorted by the coordinate conversion equations of the equations (1) and (2), and the synchronization phase θ is also a parameter and is represented by the following equation.

g (s, t, θ) = f {s + αsin (ωs + θ), t + βsin (ωs + θ)} (3) Now, when the equation (3) is partially differentiated with respect to the synchronization phase θ, And this partial derivative for the output image coordinates s, t is Becomes From equations (4) and (5), Is. From the above equation, equation (6), and equation (4), A linear form with unknown variables α and β as variables is obtained. Since the expression (8) holds at all points on the output image coordinates (s, t), observation equations for estimating α and β are obtained for the number of pixels. The vibration amplitudes α and β can be estimated with the least error by the method of least squares.

In actual image processing, the differential operation is approximated by a difference operation. The roughest approximation is Can be used. Expression (11) is the calculation of the difference between the two images obtained by slightly changing the synchronization phase Δθ.

From the above, for the calculation of α and β estimation, rewrite equation (8) as a _i α + b _i β = c _i (12) Is determined by Here, i is a serial number of pixels, and N is the total number of pixels used for estimation.

The vibration estimation method is also established in different scanning imaging modes, that is, in a high-speed scanning mode in which a scanned image is displayed on a monitor. In the high-speed scan mode, one frame is normally imaged in synchronization with the power supply frequency, and phase lock is applied at the start of scanning one frame. Therefore, equations (1) and (2) that represent distortion, and equations for estimating the vibration amplitudes α and β are used. (8)
Is as follows, but the basic distortion estimation procedure is the same as x = s + αsin (vt + θ) (14) Y = t + βsin (vt + θ) (15) Is. Here, v is the vibration frequency viewed in the vertical direction.

Even when the power supply synchronization at the start of electron beam scanning is performed in a fixed phase, the output image data obtained by shifting the field of view by a small amount by shifting the coordinates of the output image data in the opposite direction by the equivalent amount is effectively shifted in output image. Can be obtained. This is because, for example, in the equation (3), the output image g ′ (s, t, θ) obtained by moving the visual field in the scanning direction by Δd is g ′ (s, t, θ) = f {s + αsin (ωs + θ) Δd , t + βsin (ωs + θ)} (17), and the shifted output image is g ″ (s, t, θ) = g ′ (s−Δd, t, θ) = f (s + αsin (ωs + θ−ωΔd) , t + βsin (ωs + θ)} = g (s, t, θ−ωΔd) (18) In the high-speed scan mode, the field of view and the output image coordinate may be shifted in the vertical scanning direction. .

In the above description of the principle, it is assumed that the power supply magnetic field oscillation on the image is stationary for a certain period of time (for example, 2 to 3 minutes). However, when the stationary property is weak, the electron beam scanning is switched to the synchronous phase. It is preferable that the least squares method of the equation (13) is repeated twice and the stationarity is approximately satisfied.

Next, the principle of detecting the positional deviation between the frame images will be described.

Consider additive white noise as noise in the frame image. Further, the direction in which the projection data shows the unique pattern is made to coincide with the scanning vertical direction. i of the i-th frame image
The 1st scan line data is regarded as a one-dimensional signal g
_When written as _ij (t), g _ij (t) = f _i (t) + n _ij (t) (19) from the above assumption. Here, f _i (t) is a signal component and n _ij (t) is a noise component. When the number of projection lines is N, the projection data P _j (t) of the j-th frame image is calculated by using equation (1). Is represented. The above expression is a method of expressing a projection pattern peculiar to an image. The first term on the right side of the above equation is projection data of a signal component common to each frame image, and the second term is projection data of noise independent in each frame image, but the variance of the second term is the projection data. By processing Since it decreases in proportion to,
The influence of noise on P _j (t) can be reduced. Therefore, the peculiar pattern obtained by the projection process almost reflects the peculiar projection pattern of the signal itself, which enables accurate cross-correlation calculation that suppresses the influence of noise without losing the information of the signal. It is possible to prevent misdetection of the positional deviation.

Next, the effect of reducing the calculation amount according to the present invention will be described. Total number of operations required for two-dimensional cross-correlation calculation of chip size M × N C _2D
Is C _2D = (2MN−1) × m. Here, m is a representative score of the correlation calculation. On the other hand, when the data at the point M is projected in the section N and the correlation is calculated with the line segment length K, the total number of operations C _P is C _P = (N−1) M + (2K−1) × m. If you take the ratio of both Becomes As an example, when calculating M = N = 100, K = 100, m = 10, Therefore, the calculation amount of 1/16 is sufficient for the two-dimensional cross-correlation.

Finally, the effect of applying the present invention will be described. In a scanning electron microscope with an addition writing function that has an image memory inside, it is possible to detect and correct the positional deviation of each added image before addition. As a result, it is possible to prevent blurring due to the position shift of the image after addition, so that a clear image can be obtained.
Further, in a general type scanning electron microscope (including a type having a built-in image memory), the field of view can be fixed by feeding back the detected image shift to the electron beam scanning system. As a result, for example, drift of the target image over time and deviation of the visual field of the sample in so-called stereoscopic observation in which the sample stage is tilted for observation can be prevented, and the burden on the observer for adjusting the visual field can be reduced.

〔Example〕

The first embodiment of the present invention will be described below with reference to FIG. The phase of the signal from the AC power supply 1 is detected by the power supply phase detection device 2, and when the phase is 0, a synchronization signal is sent to the address generation device 3. When the address generator 3 is in the scan start waiting state, it inputs the scan start phase θ and waits for a synchronization signal from the power supply phase detector 2. When receiving the synchronization signal, the address generator 3 starts the counter of the internal clock, and when the count value becomes equal to the scan start phase θ, starts generating the address sequence on the image memory, and 1
Generates addresses for lines and enters the waiting state again. As the scanning start phase θ, θ = 0 is first given by the switch 4. The address generator 3 sends a scanning start synchronizing signal to the electron beam scanning signal generator 5 at the same time as generating the address sequence. The electron beam scanning signal generator 5 uses an integration calculation circuit and is activated by a scanning start synchronization signal to generate a linear scanning signal. The scanning electron microscope 6 outputs an image signal captured in accordance with the scanning signal, and the A / D converter 7
The A / D converted data is recorded by the switch 8 in the image memory 9 at the address position from the address generator 3. Scanned image data for one image is stored in the image memory 9
When stored in, the switches 4 and 8 are switched to the positions shown in FIG. 1 and the image data is input again. That is, the scanning start phase θ is set to Δθ, and an image in which the power source magnetic field distortion is slightly shifted from the image recorded in the image memory 9 is captured. The captured image data passes through the switch 8,
Entering the adder 10, the difference from the image data read from the image memory 9 is obtained. The difference image data is multiplied by the reciprocal 1 / Δθ of the minute phase, and the value of equation (12), that is, equation (13)
C _{i at} is obtained. The above calculation and the output of C _i are performed for each clock of the address generator 3, and in synchronization with this, the scanned image data in the image memory 9 is added by the adder 1
Input to 1 and 12, the values of equation (9) and equation (10) are obtained.
However, the register 13 for one pixel is used for the equation (9) which is the differential in the scanning direction, and the shift register 14 for one scanning line is used for the calculation of the equation (10) which is the differential in the scanning vertical direction.
Is used. The coefficients of the vibration amplitudes α and β in the equation (8),
That is, a _i and b _i of the equation (12) are obtained as shown in FIG. The coefficient a _i is obtained by multiplying the output of the adder 10 by a constant ω / Δθ and subtracting it from the output of the adder 11 to obtain the cosine function table.
It is obtained by multiplying 15 by the value of cos (ωs) obtained by referring to 15 by the scanning direction output image coordinate s which is the output of the address generator 3. In the cosine function table 15, the cosine function cos (ωs) for s = 1, 2, ..., 512 is calculated in advance by the table calculation means 16, and the value is set in the table 15. The coefficient b _i is obtained by multiplying the output of the adder 12 by the reference value of the cosine function table 15. The calculation results a _i c _i , b _i c _i , between the three coefficients a _i , b _i , c _i output at each clock of the address generator 3 by the five multipliers 17 to 21.
a _i ² , a _i b _i , b _i ² are obtained, and the vibration amplitude estimation calculation means 22 performs cumulative addition of Equation (13), inverse matrix calculation, and matrix product calculation,
Vibration amplitude estimate Is required. Vibration estimate Is input to the distortion correction address calculation means 23, and the equation (1),
By the inverse function calculation of (2), the scanning line pixel coordinate sequence (1,1), (2,1), ..., (512,
It corresponds to 1), (1, 2), ..., (511,512), (512,512). The output image coordinates (s, t) are obtained, the image memory 9 is accessed, and a corrected image without distortion is output.

Next, a second embodiment of the present invention will be described with reference to FIGS. In FIG. 3, the scanning electron microscope 31 having the image memory 310 and the addition / writing function of the image receives the processing start signal, and the scanning signal digitized by the A / D converter 39 is 1 / added by the divider 318. The number of operations is performed, the obtained value and the content of the address designated by the read address control device 312 in the image memory 310 are added by the adder 319, and the result is written again at the same address in the image memory 310. It is continuously performed during the frame cycle, and this is repeated several times. On the other hand, the image shift detection device 33 receives the processing start signal and first, as an initial setting, determines the origin position in the image shift detection.
34, the line buffer 35 is generated in synchronism with the electron beam scanning of the scanning electron microscope 31, and at the same time, the data is stored in the cumulative line data register 36 and used as the base data. After the above initialization, the shift detection cycle is started. In the shift detection cycle, cumulative line data is generated by the adder 34 and the line buffer 35 in the same manner as above for the second and subsequent frame images from the start of processing, and immediately after the completion, the cross-correlation between the data and the base data is generated. Is calculated by the cross-correlation calculation device 7 while changing the read position of the line buffer 35. The calculated cross-correlation is transferred to the shift amount calculation device 38 each time, and the maximum value is detected by the comparison process. Simultaneously with the end of the cross-correlation calculation, the position from the origin that gives the maximum value is calculated,
The read address control device 31 using it as the image shift amount
According to the present embodiment, the shift correction of the image can be performed by the real-time processing synchronized with the electron beam scanning. Further, there is an effect that the image shift correction can be performed by a simple process of changing the read address of the image memory. Since all the processing is performed in synchronization with the processing cycle of the scanning electron microscope 31, the processing other than the cumulative data generation is completed within a time period of one frame (a blanking time of about 1 msec in the case of TV rate). . Further, since the addition writing process of the scanning electron microscope 31 and the shift detection process of the image shift detection device 33 proceed completely in parallel, the shift detection result in each frame cycle cannot be immediately reflected in the frame. Therefore, in this embodiment, the detection result of the immediately preceding cycle is used.

FIG. 4 shows a scanning electron microscope having no image memory, in which the image shift is corrected by directly changing the scanning control signal of the electron beam scanning device 313 instead of changing the read address. Since the internal signals of the scanning electron microscope 32 are all analog, they are digitized by the A / D converter 316, and the image shift detection device 33
Sent to Further, the shift amount obtained by the device is data-converted by the shift amount-scanning position conversion device 317 and sent to the electron beam scanning device 313 through the D / A converter to change the scanning position. According to the present embodiment, not only the vibration of the image can be removed, but also the visual field deviation of the observation object can be prevented.

In each frame cycle described above, after the image shift detection process is completed, the position shift from the previous frame image can be detected by transferring the contents of the line buffer to the cumulative line data register. Although this method has a risk of error accumulation, it is effective when the appearance of the image changes with time. Further, in FIG. 3, an image memory is provided as a buffer between the A / D converter 39 and the adder 319 (used as an image buffer) to temporarily store the input image data, and the image shift arithmetic unit in the same frame cycle.
By changing the read address of the image buffer according to the shift amount calculated in 33 and adding the read address to the image memory, the image shift correction can be performed without the time delay of the positional deviation detection, and the deterioration of accuracy due to the delay can be prevented.

〔The invention's effect〕

According to the present invention, in a scanning electron microscope, shape distortion that slowly changes on an image due to a power supply stray magnetic field can be easily calculated from two images obtained by slightly displacing the distortion. Scanning electron suitable for obtaining a scan image with less distortion of image pattern shape and semiconductor line width measurement error due to stray magnetic field from power distribution board or cable, etc., because it can be estimated by image processing and correct the distorted image. A microscope can be provided.

In addition, even if the S / N is poor and the observation target has low texture,
If the projection data in a certain direction shows a pattern peculiar to the image, by using the projection data, S /
Since N can be improved and one-dimensional processing is performed, it is possible to provide an image correction device that can perform vibration correction with low calculation amount and high accuracy.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of the first embodiment of the present invention, FIG. 2 is an explanatory diagram of image pickup target coordinates, output image coordinates and distortion for explaining the principle of the present invention, and FIG. Second of the present invention
FIG. 4 is an overall configuration diagram of an image correction device of a scanning electron microscope with a built-in image memory, and FIG. 4 is an overall configuration diagram of an image correction device of a scanning electron microscope having no image memory according to the present invention.

Front page continuation (72) Inventor Fuminobu Furumura 1099, Ozenji, Aso-ku, Kawasaki City, Hitachi, Ltd. System Development Laboratory (72) Inventor, Takashi Iizumi 882, Ichi, Katsuta City, Naka Factory, Hitachi Ltd.

Claims (2)

(57) [Claims] 1. A scanning electron microscope apparatus comprising: a power source phase detecting means; an electron beam scanning control means for activating an electron beam scanning start when the detected phase matches a preset phase; An image memory for storing the obtained scan image data, a strain displacement image input means for inputting scan image data of the same field of view by slightly changing the set phase, image data of the image memory and the strain displacement image. From the subtraction means for obtaining the difference between the image data input by the input means, the differential operation means for obtaining the differential value of the image data in the scanning direction and the scanning vertical direction, and the difference image by the subtraction means and the differential image by the differential operation means. Processing means for estimating the power source magnetic field distortion on the image by the least squares method based on the difference equation, and the image on the image memory by the estimated distortion Image correction apparatus of a scanning electron microscope, characterized by comprising an image distortion correcting means for correcting the over data. 2. The distortion / displacement image input means, while the set phase of the electron beam scanning control means is fixed, means for moving a field of view by a small amount to input a scanning image, and the coordinates of the input image are shifted by a small amount. An image correction apparatus for a scanning electron microscope according to claim 1, further comprising: