JPH08212951A - Automatic axis adjusting device in scanning electron microscope - Google Patents

Abstract

PURPOSE: To perform axial adjustment so that an electron beam, obtained from an electron gun, automatically passes the center of a lens system without manual operation by an operator, while displaying a preset value on a CRT. CONSTITUTION: A filament 16 is heated to obtain a suitable electron beam, but the electron beam 25 not necessarily passes the center of a lens system, and an axis is automatically adjusted so that the electron beam passes the center of lens system by a gun 1 alignment coil 19 and a gun 2 alignment coil 20 controlled by a gun alignment coil control circuit 7. Accordingly, in this invention, just after mounting a filament, and when markadly changed an acceleration voltage value, even when the electron beam is deviated from the center of the lens system, by automatically performing adjusting, dispersion instantaneous further due to an individual difference is eliminated, to perform optimum axial adjustment, further to display a preset value in a CRT, so as to result in improving operability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic axis adjusting device for a scanning electron microscope, which performs axis adjustment so that an electron beam passes through the center of a lens system.

[0002]

2. Description of the Related Art A scanning electron microscope applies a high voltage generated by a high voltage generating circuit to an electron gun to generate an electron beam, and narrows the electron beam with a converging (condenser) lens.
The sample is scanned by the action of the deflection coil controlled by the scanning oscillation circuit and the deflection control circuit. Information signals such as secondary electrons and reflected electrons generated from the sample when the electron beam scans the sample are detected by the detector. The detected analog information signal is directly input to the display device and used for image display, or is converted into a digital signal by the A / D converter and then stored in the frame memory for still image display or image filing. Used.

The electron beam emitted from the electron gun uses a filament. However, if the filament is attached or the acceleration voltage is changed significantly, the electron beam deviates from the center of the lens system and an optimum information signal is obtained. You will not be able to get it.
Therefore, in the prior art, the two gun alignment coils are manually adjusted using a knob so that the electron beam passes through the center of the lens system.

[0004]

In the above-mentioned prior art, while manually turning the two knobs of the gun alignment coils X and Y, the adjustment of the gun 1 and gun 2 alignment coils is alternately repeated so that the screen becomes brightest. It takes time to make adjustments.

Further, since the sensory elements are large, there are variations in adjustment due to individual differences. Further, since the adjusted current value cannot be visually observed, there is a problem that it cannot be judged whether the value is really proper.

In order to solve the above-mentioned problems of the prior art, an object of the present invention is to automatically adjust the axis so that the electron beam passes through the center of the lens system even if the operator does not operate it. An object of the present invention is to provide an automatic axis adjusting device for a scanning electron microscope, which improves operability by displaying on a CRT (display device).

[0007]

In order to achieve the above object, according to the present invention, a set of four coils in the upper part of the mirror body is provided in a total of eight in two steps, and the upper gun 1 (tilt) alignment. The coil and the lower gun 2 (horizontal) alignment coil are arranged so as to face each other around the electron beam, and all of these coils are controlled by the CPU.

The control of the coil is performed by changing the magnetic field by changing the currents flowing in the X and Y alignment coils of the guns 1 and 2, and changing the direction of the electron beam. More specifically, the electron beam off the center of the gun 1 is
The amount of the electron beam that reaches the sample is changed by correcting the electron beam by using the gun 2 so that the electron beam is directly directed downward by the gun 2. Automatic axis control is realized by controlling the current value of the gun alignment coil so that the detected signal is maximized.

[0009]

[Operation] By combining this automatic axis adjustment function with other automatic control functions (functions such as automatic brightness correction and automatic focus correction), the operability of the scanning electron microscope can be further improved. It becomes possible (even a novice operator can operate the scanning electron microscope).

[0010]

Embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram showing the arrangement of an embodiment of a scanning electron microscope according to the present invention. Central processing unit 1 (hereinafter CP
(Referred to as U) controls all control circuits of the scanning electron microscope, controls the high voltage control circuit 3 and the filament circuit 4 via the D / A converter 2, and provides feedback from the emission current detector 6. By obtaining a signal, the filament 16 is heated and an appropriate electron beam is obtained. Further, Wehnelt 1 controlled by the bias circuit 5
Electrons are narrowed down by the electrodes of 7 and the anode 18.

The electron beam 25 obtained as described above does not always pass through the center of the lens system, and the electron beam is moved by the gun 1 alignment coil 19 and the gun 2 alignment coil 20 controlled by the gun alignment coil control circuit 7. Is corrected to pass through the center of.

The axis-corrected electron beam is narrowed down by two converging (condensing) lenses 21 and 22 controlled by the converging lens control circuit 8. The focused electron beam is
The sample 26 is scanned by the action of the scanning oscillation circuit 10 that controls the scanning speed and the like, the deflection control circuit 11 that deflects the electron beam, and the deflection coil 24.

By scanning the electron beam on the sample 26, an information signal 28 such as secondary electrons and reflected electrons is emitted from the sample 26.
Is generated and this information signal is detected by the detector 15. The information signal is displayed on the CRT 13 by the display circuit 14 or converted into a digital signal (image data) and stored in the frame memory.

The display circuit 14 has a function of displaying a menu in addition to displaying the information signal, and displays various control data (gun alignment coil setting value etc.) performed in the above in an appropriate menu. (Details will be described with reference to FIG. 3).

Conventionally, the electron beam generated from the filament cannot be obtained an optimum information signal because the electron beam deviates from the center of the lens system due to a large change in the mounting method of the filament and a large change in the acceleration voltage value. To correct this, while manually turning the two knobs of the gun alignment coils X and Y, the gun 1 and gun 2 alignment coils are alternately repeated so that the screen becomes brightest (the information signal becomes maximum). Although the adjustment is performed, there is a problem that the adjustment takes time and there are variations in the adjustment due to individual differences because the sensory element is large.

In this embodiment, the manual operation described above is CP
U is used for automatic control. The control method is performed by changing the magnetic field by changing the currents flowing in the X and Y coils of the gun 1 and 2 alignment and changing the direction of the electron beam. More detailed description will be given with reference to the diagram shown in the lower part of FIG. 1. The gun 1 alignment coil 19 corrects the electron beam 25 deviated from the center toward the center by θ °, and the gun 2 alignment coil 20 is used to correct the electron beam. By correcting by θ '° so that it is directly below, the amount of electron beam reaching the sample is changed, and the current value of the gun alignment coil is maximized so that the detection signal obtained from the detector attached at each location is maximized. The automatic axis adjustment is realized by controlling.

The detectors attached to the respective parts will be further described. The first detector 9 attached to the diaphragm and
The second detector 12 mounted on the sample table, and the sample 26
Information signal 28 obtained by scanning the electron beam on
The information signal detector 15 for detecting the detection signal is provided, and the CPU reads and controls the detection signal obtained from each detector by changing the current amount of the gun alignment coil by the method described above.
Although the detailed control method will be described with reference to FIG. 2, the information obtained from the detectors may be used individually as a judgment criterion, or the combination of the detected signals may be used to make a comprehensive judgment for optimum gun alignment. The current value of the coil may be determined.

Although it is the timing for executing the automatic axis adjustment, it may be executed by selecting a switch or the like, or may be executed in conjunction with the factor that the electron beam deviates from the lens system when the acceleration voltage is applied or when the acceleration voltage value is changed. You may.

Further, an example of the algorithm will be described with reference to the flowchart of FIG.

In this example, the information signal 28 is transferred to the information signal detector 15
Is an example in which the contrast data is detected and the contrast data is measured by the display circuit.

(1) As the initial processing, the data of the gun 1 and gun 2 alignment coils are saved in order to initialize each flag and to restore the state before execution when an error occurs or is canceled, gun 1, gun 2 alignment coil. The reference coordinates (X, Y) of the respective alignments are set to the center ($ 80, $ 80) of the alignment coil current. (X
== $ 80, Y = $ 80 will be the reference coordinates in the initial stage)
Also, the change interval is set to the initial value of $ 40 (to measure the entire area).

(2) Set the optimum scan speed.

(3) The gun 2 is corrected. The current value of the alignment coils X and Y is set for each change interval with the reference coordinate as the center.

(4) After one scan, the contrast data is measured and the data is stored. At that time, it is checked whether the contrast data has the maximum value. The X and Y coordinates for which the maximum value is found become the reference coordinates for the next step (in the example of FIG. 2, X = $ 80 and Y = $ 40 indicate the next reference coordinates).

(5) Measurement at 25 points per step ((3),
(4)) is repeated.

(6) After the measurement of the gun 2 is completed, the same process as the gun 2 is performed on the gun 1.

(7) The above processes (3) to (6) are set as one set and are repeated for a prescribed number of steps. then,
The change interval is reduced to half each time and more detailed measurement is performed. (In Figure 2, the first step is the reference coordinate ($ 80, $
Since the maximum value was obtained at the coordinate ($ 80, $ 40) at the change interval $ 40 in 80), the reference coordinate in the next two steps becomes the change interval $ 20 at ($ 80, $ 40). In the case of 3 steps, an example is shown in which 25 points are measured with a change interval of $ 10 centering on the maximum value ($ 80, $ 60) of 2 steps. (8) As post-processing, it is determined that the X and Y points of the last detected maximum contrast data are the optimum current values of the gun alignment coil, the gun 1 and the gun 2 alignment coils are set, and the process ends.

With the above processing, it is possible to find the optimum current value of the gun alignment coil.

In addition to the method described above, the number of measurement points per step may be increased from 25 points to 81 points for measurement, and the number of steps may be set to 5 steps instead of 3.
It is also possible to increase the accuracy of axis adjustment.

Next, after the optimum current values of the gun 1 alignment coils X and Y and the gun 2 alignment coils X and Y are found, the state of the set current value is displayed on the display circuit 1.
4, a display method using the CRT 13 will be described with reference to FIG. The left part of FIG. 3 shows a cross-sectional view of the actual configuration of the gun alignment coil, which is a state in which the electron beam passes.
The right part is a menu that schematically displays the current value set in the gun alignment coil at that time. The menu has X-coordinates and Y-coordinates, and the point where one line is drawn from the center of each of the X- and Y-coordinates and intersects is the reference position (the most appropriate position). The X and Y data set for each coil are displayed by cross arrows or the like, and the adjusted set values can be confirmed at a glance (the optimum state is when the reference position and the cross arrows overlap). The display timing of the menu may be displayed immediately after the automatic axis adjustment, or may be displayed by selecting a switch or the like. Also, this display menu can be used not only for automatic axis adjustment but also for manual adjustment.

According to the present invention, the operability can be greatly improved by automatically adjusting the electron beam so that the electron beam passes through the center of the lens system even if the operator does not operate by the above method. Can help you.

[0033]

EFFECTS OF THE INVENTION According to the present invention, immediately after the attachment of the filament, and when the accelerating voltage value is changed significantly, the electron beam is automatically adjusted even if it is out of the center of the lens system. Optimal axis adjustment with no variations brings about great improvement in operability.

[Brief description of drawings]

FIG. 1 shows a configuration diagram of a scanning electron microscope.

FIG. 2 shows a flowchart of an automatic axis adjustment method.

FIG. 3 shows an example of a set value state menu.

[Explanation of symbols]

1 ... CPU, 2 ... D / A converter, 3 ... High-voltage control circuit, 4
... Filament circuit, 5 ... Bias circuit, 6 ... Emission current detector, 7 ... Gun alignment coil control circuit, 8 ... Converging lens control circuit, 9 ... First detector, 10 ...
Scan oscillation circuit, 11 ... Deflection control circuit, 12 ... Second detector, 13 ... CRT, 14 ... Display circuit, 15 ... Information signal detector, 16 ... Filament, 17 ... Wehnelt, 18
… Anode, 19… Gun 1 alignment coil, 20…
Gun 2 Alignment coil, 21 ... Convergence 1 (condenser) lens, 22 ... Convergence 2 (condenser) lens, 23
... diaphragm, 24 ... deflection coil, 25 ... electron beam, 26 ... sample, 27 ... sample stand, 28 ... information signal.

Claims (4)

[Claims] 1. An electron gun, a gun alignment coil for correcting an electron beam generated from the electron gun (filament) so as to pass through the center of a lens system, a converging (condensing) lens for narrowing the electron beam, and the converging. Scanning electron microscope equipped with a deflection coil for scanning an electron beam converged by a lens on a sample, a detector for detecting an information signal output from the sample during scanning, and a display circuit for displaying the information signal on a CRT. And, in the device that automatically adjusts the axis so that the electron beam passes through the center of the lens system and goes directly to the sample using the gun alignment coil, the information signal is detected as the criterion for determining the optimum axis position. An automatic axis adjusting device for a scanning electron microscope, characterized in that it is processed by using a signal obtained from the instrument. 2. The method according to claim 1, wherein the criterion for determining the optimum axis position is processed by using a signal obtained by using one of the detectors installed. Automatic axis adjustment device for scanning electron microscope. 3. A scanning electron microscope according to claim 1, wherein an optimal electron beam path is comprehensively determined by using signals obtained from a plurality of detectors as criteria for determining the optimal axis position. Automatic axis adjustment device. 4. The means according to claim 1, after calculating the optimum X and Y currents of the gun 1 alignment coil and X and Y current values of the gun 2 alignment coil, and displaying the X and Y coordinates and the reference position on the CRT. An automatic axis adjusting device for a scanning electron microscope, comprising: