JPH09306407A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an equipment, which can detect the optimal condition of a focal point of a lens so as to accurately perform the automatic focal point control in a short time, by providing a deflecting signal generating means, which is formed of an oscillator a scanning counter, a conversion table and a D/A converter. SOLUTION: Electron beam 2 emitted from an electron gun 1 is throttled narrow by an electron lens 3. The electron lens 3 has a lens working function of a magnetic field to be generated by flowing the current in a coil, and an electron lens control circuit 10 controls the current of the coil so that the electron lens 3 works at a focal distance indicated by a control CPU 20. The electron beam 2, which is throttled narrow by the electron lens 3, is deflected by the two-dimensional scanning in a direction X and a direction Y of deflecting units 4, 5. Deflecting amplifiers 11, 12 controls the current to be flowed to coils of the deflecting units 4, 5. When a sample is irradiated with the electron beam 2, secondary grains 8 such as secondary electron is generated in response to the shape and the material of the sample. The secondary grains 8 are detected by a secondary grain detection unit 9 and amplified, and stored in an image memory 14.

Description

Detailed Description of the Invention

[0001]

[0001] The present invention relates to a scanning electron microscope.

[0002]

2. Description of the Related Art An electron microscope is used in many research and development fields as an effective tool for observing a fine structure of a sample. FIG. 2 shows the configuration of a conventional general scanning electron microscope, and the operation will be briefly described. The electron beam 2 emitted from the electron gun 1 is narrowed down by the electron lens 3, and the deflector 4 and the deflector 5 perform two-dimensional scanning deflection to irradiate the sample 6. When the sample 6 is irradiated with the electron beam 2, secondary particles 8 such as secondary electrons according to the shape and material of the sample 6 are generated.
After the secondary particles 8 are detected and amplified by the secondary particle detector 9, A /
The D converter 13 converts the digital value into image data and stores it in the image memory 14. The image data stored in the image memory 14 is converted into a video signal by the D / A converter 15 and displayed on the display 16 as a scanning electron microscope image. The image memory control circuit 17 writes in the image memory 14 synchronized with the scanning of the electron beam 2 and the image memory 1 synchronized with the video signal.
The address bus, the data bus, and the control signal of the image memory 14 are controlled so that 4 is read out. The scanning deflection signal generation circuit 27 supplies the image memory control circuit 17 with generation of a sawtooth wave which is a two-dimensional deflection scanning signal given to the deflection amplifiers 11 and 12 and a write address of the image memory 14.

In such a scanning electron microscope, the autofocus control for automatically setting the focus condition of the electron lens 3 to an optimum value is performed by scanning a plurality of frames in which the condition of the electron lens 3 is changed. Then, the obtained secondary particle detection signal is evaluated by the focus evaluation value detection circuit 19, and the optimum value is set as the condition of the electron lens. As an example, there is JP-A-5-190132 "Automatic focusing method for a scanning electron microscope".

[0004]

In the conventional scanning electron microscope, the scanning deflection signal generation circuit 27 generates a deflection signal for scanning a quadrangular plane, and takes in scanning electron microscope images of a plurality of frames. There is a problem that it takes several tens of seconds until the automatic focus control ends. Also, in order to reduce the processing time, the surface is not scanned,
There are methods to perform scanning such as cross-shaped scanning and spiral scanning, but the optimum focus is required because a signal generation circuit for special scanning is required, or the scanning position of the electron beam only depends on the state of the sample. There was a problem that could not be detected.

An object of the present invention is to provide an electron beam scanning signal generating means and an automatic focus control method capable of accurately performing automatic focus control for detecting the optimum condition of the focus of an electron lens in a short time.
An object of the present invention is to provide a scanning electron microscope capable of obtaining a comfortable operating environment.

[0006]

In order to achieve the above object, the present invention comprises a deflection signal generating means composed of an oscillator, a scanning counter, a conversion table and a D / A converter, and a small amount of image data. It has a scanning mode of multiple electron beams of low resolution scanning for obtaining high resolution and high resolution scanning for obtaining a large amount of image data, and automatic focus control is performed again by high resolution scanning after performing automatic focus control by low resolution scanning. I do. Further, the oscillator oscillates at an arbitrary oscillation frequency so that the scanning speed is such that the secondary particles can be detected efficiently.
Further, the scanning counter can set the counting range according to the scanning mode and the scanning range. Furthermore, the conversion table is rewritable and conversion data is set according to the electron beam scanning mode.

[0007]

FIG. 1 is a block diagram of a scanning electron microscope which is an embodiment of the present invention. The electron beam 2 emitted from the electron gun 1 is narrowed down by the electron lens 3. Electronic lens 3
Uses the lens action of the magnetic field generated by passing a current through the coil, and the electron lens control circuit 10 controls the current of the coil so that the lens action of the electron lens 3 becomes the focal length instructed by the control CPU 20. To do. Deflector 4,
Reference numeral 5 designates the electron beam 2 narrowed down by the electron lens 3 in the X direction and the Y direction.
Scan and deflect in two dimensions. The deflectors 4 and 5 also deflect the electron beam 2 by the magnetic field generated by flowing a current through the coils, and the deflection amplifiers 11 and 12 control the currents flowing through the coils of the deflectors 4 and 5. When the sample 6 is irradiated with the electron beam 2, secondary particles 8 such as secondary electrons are generated depending on the shape and material of the sample 6. The secondary particles 8 are detected and amplified by the secondary particle detector 9, converted into digital value image data by the A / D converter 13, and stored in the image memory 14. The image data read from the image memory 14 is the D / A converter 15.
Is converted into a video signal, and a scanning electron microscope image is displayed on the display 16. The image memory 14 is composed of a memory having a maximum size of 1,024 pixels × 1,024 pixels of image data to be stored and a size of 1 M (1,048,576) bytes when the gradation is 8 bits. The image memory control circuit 17 writes to the address synchronized with the scanning signal of the electron beam 2 generated by the deflection signal generation means 21 and the video synchronization signal generation circuit 18.
The address bus of the image memory 14, the data bus,
Control the control signal. The deflection signal generating means 21 includes an oscillator 22, a scanning counter 23, a conversion table 24, and
It is composed of D / A converters 25 and 26, generates an electron beam scanning deflection signal, and supplies it to the deflection amplifiers 11 and 12. The focus evaluation value detection circuit 19 detects the evaluation value of the focus state from the obtained image data.

FIG. 3 shows a configuration example of the deflection signal generating means 21. The oscillator 22 outputs a pulse generated by dividing the pulse generated by the basic oscillator 30 by the frequency divider 31 at a frequency division ratio designated by the control CPU 20. Arbitrary scanning speed can be realized by setting the frequency division ratio. The basic frequency is 20MHz,
When the number of pixels in the X direction is 1,024 pixels, the division ratio is 1, and the electron beam retrace period is 12.8 μs for 256 pixels,
The scanning time for one line is (1,024 + 256) / 20MH
The scanning speed is 64 μsec obtained by z, and the scanning speed is almost equal to the horizontal period of the video signal called NTSC. When the frequency division ratio is 313 and the blanking period is 256 pixels, 1
The line scanning time is (1,024 + 256) × 313/2
20ms obtained at 0MHz, 50H of commercial power supply
A scanning speed approximately equal to the period of z is obtained. By setting the frequency division ratio in this way, the scanning speed can be changed. The scanning counter 23 includes two sets of counters 3.
2, 33 and comparators 34, 35. Counter 33
Is an 11-bit output counter capable of counting up to 2048 in order to obtain image data having 1024 horizontal pixels and a blanking period. The counter 32 has 1024 vertical pixels.
It is a 10-bit output counter that can count up to 1024 to get pixels. When the count ranges of the counters 37 and 36 are set in the registers of the comparators 35 and 34 and the reset of the counters 33 and 32 is released, the counting of the pulses output by the oscillator 22 is started. When the output of the counter 33 reaches a value within the set counting range, the comparator 35 resets the counter 33 and counts up the counter 32. When the counter 32 reaches the value within the set counting range, the comparator 34 resets the counters 32 and 33 and starts scanning the next frame. The conversion table 24 is composed of two sets of memories 36 and 37, converts the outputs of the counters 32 and 33 and gives them to the A / D converters 25 and 26. Memory 37
Is 2048 × with 11-bit input and 10-bit output
The memory 36 has a size of 10 bits, and the memory 36 has a size of 1024 × 10 bits with 10 bits for input and 10 bits for output. The conversion data of the conversion table 24 includes an address bus, a data bus, and a control signal from the counter 23 side to control C.
The control CPU 20 writes the data by switching to the PU 20 side. D
The / A converters 25 and 26 convert the digital output of the conversion table 24 into an analog signal, and the deflection amplifier 11,
12 as a deflection signal.

FIG. 4 shows a configuration example of the focus evaluation value detection circuit 19. Here, it is determined that the optimum focus is obtained when the total sum of the absolute values of the differential values of the image data is maximum.
The subtractor 41 subtracts the image data previously obtained from the image data obtained by the A / D converter 13 and delayed by the delay register 40.
To obtain the differential value. Next, in the absolute value circuit 42, when the differential value is negative, the two's complement is calculated and set to the absolute value. Further, the adder 43 and the register 44 determine the total sum.
The control CPU 20 reads the evaluation value thus obtained.
The delay of the delay register 40 is set to one line scanning or more, and Y
Alternatively, the differential value in the axial direction may be obtained, and the subtractor 41 may be a spatial filter that obtains the spatial differential. The automatic focus control with the above configuration will be described below.

In the first automatic focus control example, first, in the memories 36 and 37 of the conversion table 24, the outputs Xo and Yo are respectively input to the inputs Xi and Yi as shown in FIGS. 5A and 5B. The conversion data is set to be N or M times larger, and the count ranges of the counters 34 and 35 of the scanning counter 23 are set to 10
When 24 pixels × 1024 pixels are set to be 1 / N and 1 / M, respectively, the electron beam scanning is a low-resolution whole-surface scanning thinned as shown in FIG. 5C. If the frequency of the oscillator 19 is the same, the time for scanning one frame is 1 /
(N × M). By this low resolution scanning, a plurality of frames in which the excitation condition of the electron lens 3 is changed are scanned, the focus evaluation value detection circuit 19 detects the focus evaluation value, and the optimum value of the excitation condition of the electron lens 3 is obtained. .
Next, in the memories 36 and 37 of the conversion table 24 shown in FIG.
The conversion data for performing high-resolution full-screen scanning as shown in (a) and (b) is set. Then, the focus evaluation value detection circuit 1 is scanned by scanning a plurality of frames in which the excitation condition of the electron lens 3 is changed near the optimum value obtained by the low resolution scanning.
The focus evaluation value is detected by 9 and the optimum value of the excitation condition of the electron lens 3 is obtained. With such a method, accurate automatic focus control can be performed in a short time. Here, the two-step control has been described, but it is also possible to perform multi-step fine control.

When high-speed scanning is performed, the irradiation amount of the electron beam per unit area of the sample decreases, and the influence of noise on the secondary particle detection signal increases. In order to obtain high quality image data, it is necessary to scan at low speed. In the second example of automatic focus control, first, a plurality of frames in which the excitation conditions of the electron lens 3 are changed by high-speed scanning are scanned,
The focus evaluation value detection circuit 19 detects the focus evaluation value, and obtains the optimum value of the excitation condition of the electron lens 3. next,
Scanning is performed at a low speed to scan a plurality of frames in which the excitation condition of the electron lens 3 is changed in the vicinity of the optimum value obtained by the high-speed scanning, and the focus evaluation value detection circuit 19 determines the focus evaluation value. Then, the optimum value of the excitation condition of the electron lens 3 is obtained. With such a method, accurate automatic focus control can be performed in a short time. The control of the scanning speed can be easily realized by the control CPU 20 setting the frequency division ratio of the frequency divider 31 of the oscillator 22 of the deflection signal generating means 21. Further, the optimum value of the scanning speed at which an efficient signal can be input to the focus evaluation value detection circuit 19 can be obtained from the evaluation value.

When the automatic focus control is performed when the sample 6 to be observed has a height difference, there is a problem that the portion to be observed is out of focus. The third example of automatic focus control is an example in which the optimum focus of the portion to be observed is obtained. When a certain amount of image is obtained, as shown by the thick line on the image displayed on the display 16 as shown in FIG.
Is specified. Then, only the range 50 may be scanned with an electron beam to perform automatic focus control. this is,
As shown in FIG. 8, the memories 36 and 37 of the conversion table 24 are
The conversion data is set to scan only a specified range starting from A and B, and the counters 34 and 3 of the scanning counter 23 are scanned.
This can be realized by setting the counting range of 5 to a range in which the blanking period is added to the effective scanning periods C and D. If the length of the scanning range in the X direction is 1 / N and the length in the Y direction is 1 / M, the scanning time for capturing an image of one frame is 1 / (N except the blanking period.
XM). By such a method, it becomes possible to shorten the automatic focus control and focus the portion to be observed with high accuracy.

The fourth example of automatic focus control is a method of performing not only surface scanning but also arbitrary line scanning. As shown in FIG. 9, the output of the counter 33 of the scanning counter 23 is input to both of the memories 37 and 36 of the conversion table 24, and the conversion data of the memories 37 and 36 of the conversion table 24 is converted into FIG.
The values shown in (a) and (b) are set. Then, the scanning with the electron beam has a diameter of 1 / X in the X direction as shown in FIG.
Draw a circle whose diameter in the N and Y directions is 1 / M. Automatic focus control can also be performed by such scanning. By the conversion data of the conversion table 24, it is possible to easily realize the figure 8 scanning and the spiral scanning. It is also possible to select the scanning mode by increasing the size of the conversion table 24 and writing conversion data of some scanning modes in advance to control the address. Further, the counters 33, 3 of the scanning counter 23
Both outputs of 2 are stored in the memories 37 and 36 of the conversion table 24.
To the memory 37, 36 of the conversion table 24.
If the size of the above is set to a corresponding size, not only the scanning of one line but also the complicated surface scanning can be realized.

[0014]

According to the present invention, it is possible to provide a scanning electron microscope in which automatic focus control of an electron lens can be accurately performed in a short time and a comfortable operating environment can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of a scanning electron microscope according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional scanning electron microscope.

FIG. 3 is a block diagram of deflection signal generating means.

FIG. 4 is a block diagram of focus evaluation value detection means.

FIG. 5 is an explanatory diagram showing a conversion table for low resolution scanning and scanning with an electron beam.

FIG. 6 is an explanatory diagram showing a conversion table for high resolution scanning and scanning with an electron beam.

FIG. 7 is an explanatory diagram showing designation of a scanning range.

FIG. 8 is an explanatory view showing a conversion table of designated range scanning and scanning of an electron beam.

FIG. 9 is a block diagram showing a configuration example of a deflection signal generating means for circular scanning.

FIG. 10 is an explanatory diagram showing a conversion table for circular scanning and electron beam scanning.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Electron beam, 3 ... Electron lens, 4, 5 ... Deflector, 6 ... Sample, 7 ... Sample stand, 8 ... Secondary particle, 9 ... Secondary particle detector, 10 ... Lens control circuit , 11, 12 ... Deflection amplifier, 13 ... A / D converter, 14 ... Image memory, 15
... D / A converter, 16 ... Display, 17 ... Image memory control circuit, 18 ... Video synchronization signal generation circuit, 19 ... Focus evaluation value detection circuit, 20 ... Control CPU, 21 ... Deflection signal generation means, 22 ... Oscillator, 23 ... Scan counter, 24 ... Conversion table, 25, 26 ... D / A converter.

Claims (8)

[Claims] 1. An electron gun for emitting an electron beam, an electron lens for narrowing down the electron beam, a deflector for two-dimensionally scanning the electron beam on a sample, and detecting secondary particles generated from the sample. A secondary particle detector, an A / D converter that converts a detection signal into digital image data, an image memory that stores the image data, and a D that converts the image data read from the image memory into a video signal. A scanning electron microscope including an A / A converter and a display for displaying an image, including a deflection signal generating unit including an oscillator, a scanning counter, a conversion table, and the D / A converter. Characteristic scanning electron microscope. 2. The deflection signal generating means according to claim 1, having a plurality of electron beam scanning modes of low resolution scanning for obtaining a small amount of image data and high resolution scanning for obtaining a large amount of image data. Scanning electron microscope that selects low-resolution and high-resolution scanning signals according to the conversion data of the conversion table of and supplies them to the deflector. 3. A scanning electron microscope according to claim 2, wherein automatic focus control is performed by low resolution scanning, and then automatic focus control is performed again by high resolution scanning. 4. The scanning electron microscope according to claim 1, wherein the conversion table of the deflection signal generating means is rewritable and any conversion data is rewritten according to a scanning mode of the electron beam. 5. The scanning electron microscope according to claim 1, wherein the oscillation frequency of the oscillator of the deflection signal generating means can be set, and the scanning speed of the electron beam can be set to an arbitrary speed. 6. The scanning electron microscope according to claim 5, wherein automatic focus control is performed by high-speed scanning, and then automatic focus control is performed again by low-speed scanning. 7. A scanning electron microscope according to claim 1, wherein the count range of the scan counter of the deflection signal generating means can be set and the scan range of the electron beam can be set to an arbitrary range. 8. A scanning electron microscope according to claim 7, wherein the electron beam is scanned only in an arbitrary range and automatic focus control is performed.