JP2647949B2 - Scanning electron microscope alignment equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] While stopping a scanning current flowing through a scanning coil, superimposing the scanning current on an alignment coil, an electron beam emitted from an electron gun is scanned on an objective lens aperture. The present invention relates to an alignment apparatus for a scanning electron microscope, which displays an image obtained on a cathode ray tube and corrects the alignment.

[Prior Art] FIG. 3 is a diagram showing an outline of a configuration between an electron gun and an objective lens stop of a scanning electron microscope, and FIG. 4 is a diagram for explaining an alignment function of the electron gun.

The space between the electron gun and the objective lens aperture of the scanning electron microscope is configured as shown in FIG. The field emission electron gun 51 used in such a scanning electron microscope or the like includes an emitter, an extraction electrode, and an acceleration electrode. The lens barrel includes a focusing lens 53 for controlling the probe current, an objective lens aperture 54 for determining the opening angle of the probe, and the like. In the scanning electron microscope having such a configuration, when the acceleration voltage of the field emission electron gun 51 is switched,
When the excitation of the focusing lens 53 is changed, the deviation of the emitted electron beam due to a deviation from the mechanical axis between the emitter, the extraction electrode, and the accelerating electrode or a mechanical deviation between the electron gun 51 and the focusing lens 53. An axis shift may occur, and the intensity of the electron beam passing through the objective lens diaphragm 54 may decrease. The alignment coil 52 is provided between the electron gun 51 and the focusing lens 53 in order to electrically correct this axis shift, and corrects and controls the current flowing through the alignment coil 52 by operating means such as a volume. Thus, the alignment as shown in FIG. 4 is performed.

The configuration of the apparatus for observing an enlarged image of the sample will be described with reference to FIG. 1. A scanning coil 5 is provided below the objective lens aperture 4, and a scanning signal generated by a scanning power supply 13 is supplied. . Then, the electron beam passing through the scanning coil 5 is focused by the objective lens 6 and scanned on the sample 7. The secondary electron detector 8 detects secondary electrons generated from the sample 7 by scanning the electron beam,
In the scanning electron microscope, the detection signal is supplied to the cathode ray tube 9 which is scanned by a signal synchronized with the scanning signal of the electron beam, and an enlarged image of the sample is displayed.

By the way, in the scanning electron microscope having the above-described configuration, when monitoring the shape and brightness distribution (emission pattern) of the electron beam emitted from the electron gun 1, the current flowing through the scanning coil 5 is stopped, and The scanning current is superimposed on the current flowing in the alignment coil 2, and the electron beam emitted from the electron gun 1 is scanned on the objective lens diaphragm 4. Then, the electron beam passing through the objective lens aperture 4 is defocused by the objective lens 6, scans over the sample 7, detects secondary electrons emitted from the sample, and monitors the image on the cathode ray tube 8. ing. Or focusing lens 3
A method is used in which the emission pattern from the electron gun 1 is enlarged and projected on the objective lens aperture 4 and the alignment coil 2 scans the aperture to monitor the electron beam passing through the objective lens aperture. I have.

[Problems to be Solved by the Invention] Conventionally, electrical alignment has been performed while viewing an image obtained by the monitor as described above. Especially,
In a scanning electron microscope equipped with a field emission electron gun, when the acceleration voltage is switched, the position of the light source moves significantly. For each time, the current of the alignment coil 2 is monitored while looking at the image obtained by the monitor. Is increased or decreased to perform alignment.

However, after the completion of the alignment, the scanning current superimposed on the alignment coil 2 is stopped, and the scanning current is supplied to the scanning coil 5 to scan the electron beam. When a sample image is displayed on the tube 9, an enlarged image of the sample may be missing from the screen of the cathode ray tube 9 or may not be displayed on the screen at all, and the inaccuracy of the alignment is a problem.

By the way, since the alignment coil 2 needs to increase the deflection angle of the electron beam, a cored coil in which a coil is wound around a magnetic core is used, and the number of turns of the coil is extremely increased. In the case of a cored coil having a large number of turns as described above, a change in magnetic flux cannot catch up with a change in current flowing through the coil, causing a response delay. Therefore, as shown in FIG. 5, the scanning waveform (b) of the electron beam scanned by the alignment coil 2 having a response delay is the same as the scanning signal (a) supplied to the alignment coil 2 and the scanning signal (a).
Has a phase delay of time t with respect to the scanning signal (c) of the cathode ray tube 9 synchronized with the above.

Therefore, when electrical alignment is performed such that the alignment image obtained by the monitor is positioned at the center of the cathode ray tube, the influence of image shift due to a response delay of the alignment coil is included. Will perform the alignment correction scanning. Therefore, when the scanning by the alignment coil is stopped and the sample image is displayed by scanning the electron beam with the scanning coil having a small response delay, the axis is shifted by an image shift caused by the response delay of the alignment coil. As a result, a sample image at a desired observation position cannot be obtained, or the sample image is missing from the screen.

The present invention has been made in view of the above problems, and provides an alignment apparatus for a scanning electron microscope capable of performing accurate electrical alignment while viewing an image obtained by scanning an aperture with an alignment coil. The purpose is.

Means for Solving the Problems According to the present invention, the scanning current flowing to the scanning coil is stopped, the scanning current is superimposed on the alignment coil, and the electron beam emitted from the electron gun is scanned on the objective lens aperture. An alignment apparatus for a scanning electron microscope for displaying an image obtained by the above on a cathode ray tube and performing alignment correction, comprising: an alignment correction means for supplying the scanning current superimposed on a direct current for alignment; A correction means is provided which is switched in conjunction with switching of supply of a scanning current to the alignment coil in order to correct an error of alignment correction based on a phase delay of electron beam scanning by the alignment coil.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating the configuration of an apparatus for aligning a scanning electron microscope according to an embodiment of the present invention, and FIG. 2 is a view illustrating the operation of setting a correction signal.

In FIG. 1, 1 is an electron gun, 2 is an alignment coil, 3 is a focusing lens, 4 is an objective lens aperture, 5 is a scanning coil, 6 is an objective lens, 7 is a sample, 8 is a secondary electron detector, and 9 is a secondary electron detector. Cathode ray tube, 10 is an electron gun power supply, 11 is an alignment power supply, 12 is a focusing lens power supply, 13 is a scanning power supply, 14 is an objective lens power supply, 15a, 15b, 15c are changeover switches, 16 is a scanning terminal, and 17 is a switch control circuit. , 18 is an alignment correction circuit, 19 is an alignment error correction circuit, 20 is a constant excitation current generation circuit, and 21 is an addition circuit.

In the apparatus configured as described above, the correction signal from the alignment error correction circuit 19 is set as follows. First, the electron gun 1 of the scanning electron microscope whose alignment has been completed in advance is operated at the minimum acceleration voltage. When the alignment correction mode is set by the scanning terminal 16,
The changeover switch 15b is switched by the switch control circuit 17, so that the scan current supplied from the scan power supply 13 to the scan coil 5 is stopped, and the scan current is superimposed on the alignment current via the adder circuit 21 so that the alignment coil 2
Supplied to The excitation current of the focusing lens is set in conjunction with each acceleration voltage so that an emission pattern of an appropriate size can be observed on the cathode ray tube 9. As a result, FIG.
Is displayed as shown in FIG. However, the emission pattern A is not displayed at the center P on the cathode ray tube 9 due to the response delay of the alignment coil 2. Therefore, the current for correcting the shift of the display image based on the phase delay of the electron beam scanning from the alignment error correction circuit 19, that is, the current on the sample is added to the alignment current and the scanning current supplied to the alignment coil 2 in the alignment correction mode. The scanning center of the scanned electron beam (indicated by i in FIG. 5) is set to the scanning center of the cathode ray tube 9 (j in FIG. 5).
2), the emission pattern A is moved to the center P on the cathode ray tube 9 as shown in FIG. 2 (b). Thus, if the alignment state is correct, the sample image can be displayed at the center of the cathode ray tube 9 even when the mode is switched to the sample image display mode. The alignment error correction circuit 19 controls the switch 15b so that the error correction current value set in the circuit is superimposed on the alignment current and the scanning current in the alignment correction mode.
Is connected to the adder circuit 21 via a switch 15c that operates in conjunction with the switching of.

The alignment current and the error correction current are respectively the square root of the acceleration voltage HV. Therefore, the amount of correction (the amount of deflection of the electron beam with respect to the amount of rotation of the volume or the like) at each acceleration voltage is constant. Further, since the alignment coil 2 has a response delay, before the current value supplied to the coil changes by changing the acceleration voltage or the like,
For example, at the time of switching to the alignment correction mode, a high DC current is supplied from the constant excitation current generating circuit 20 to the alignment coil 2 through the switch 15a which is turned on for a predetermined time by the control signal of the switch control circuit 17, and The configuration is such that the alignment correction is started after a constant magnetic field is applied to the coil.

Next, an alignment method using the alignment apparatus having the above configuration will be described.

In the sample image display state, when the acceleration voltage of the field emission type electron gun is largely deflected and the position of the light source largely changes, an axis shift may occur and a sample image of a desired visual field may not be obtained. In such a case, the operation terminal 16 is first switched to the alignment correction mode. By setting the alignment correction mode by the operation terminal 16, the switch control circuit 17 turns on the switch 15a for a certain period of time,
A high direct current is supplied from the constant excitation current generating circuit 20 to the alignment coil 2 to apply a constant magnetic field to the coil 2. Thereafter, the switches 15b and 15c are turned on, and the scanning current and the error correction current are superimposed on the alignment current by the adding circuit 21 and supplied to the alignment coil 2. Then, the electron beam is scanned by the alignment coil 2, and the emission pattern A obtained by passing through the objective lens diaphragm 4 is displayed on the cathode ray tube 9. Therefore, alignment is performed such that the emission pattern A is moved to the center P on the cathode ray tube 9 by only the alignment correction circuit 18. After the completion of the alignment, when the mode is switched to the sample image display mode by the scanning terminal 16, the switch control circuit
17, the switch 15b is connected to the scanning coil 5 side, and the switch 15c is turned off. Then, a scanning current is supplied to the scanning coil 5, and the sample image of the desired visual field is displayed at the center P on the cathode ray tube 9.

The above-described embodiment is merely an embodiment of the present invention, and the present invention can be implemented with various modifications. For example, in the above-described embodiment, the DC current value for error correction set in the error correction circuit is supplied to the adding circuit during the alignment work and is superimposed on the scanning current and the alignment current. After the alignment work is completed, the DC current may be supplied to the adding circuit in conjunction with the switching of the switch 15b at the time of displaying the electron microscope image to be superimposed on the alignment current.

In the above-described embodiment, the error correction circuit is a superposition circuit of the error compensation current. However, the error correction circuit may correct an error of the alignment correction based on the phase delay by using a phase shifter. In this case, a phase shifter for shifting the scanning phase of the cathode ray tube may be provided on the cathode ray tube side.

Further, in the above-described embodiment, an emission pattern is displayed on a cathode ray tube by scanning an unfocused electron beam on an aperture,
Alignment correction is performed based on the displayed image, but the focused electron beam is scanned on the aperture to display the aperture image on the cathode ray tube based on transmission, reflection or detection of secondary electrons, and this image is displayed. The present invention can be similarly applied to the case where alignment correction is performed based on the above.

[Effects of the Invention] As is apparent from the above description, according to the present invention, the scanning current flowing through the scanning coil is stopped, the scanning current is superimposed on the alignment coil, and the electron beam emitted from the electron gun is used as an object. In an alignment apparatus of a scanning electron microscope that performs an alignment correction by displaying an image obtained by scanning on a lens aperture on a cathode ray tube, an alignment correction unit that supplies the scanning current by superimposing the scanning current on a current with an alignment direct current, Correction means that is switched in conjunction with switching of supply of a scanning current to the alignment coil is provided to correct an error in alignment correction based on a phase delay of electron beam scanning by the alignment coil with respect to scanning of the cathode ray tube. This allows the scanning center of the electron beam to coincide with the scanning center of the cathode ray tube during alignment correction. Therefore, an alignment apparatus for a scanning electron microscope that can perform accurate electrical alignment while viewing an image displayed on the cathode ray tube is realized.

[Brief description of the drawings]

FIG. 1 is a view showing an apparatus for explaining an embodiment of an alignment apparatus for a scanning electron microscope according to the present invention, FIG. 2 is a view for explaining a setting operation of a correction signal, and FIGS. Is a diagram for explaining a conventional example. 1: electron gun, 2: alignment coil 3: focusing lens, 4: objective lens aperture 5: scanning coil, 6: objective lens 7: sample, 8: secondary electron detector 9: cathode ray tube, 10: electron gun power supply 11 : Alignment power supply 12: Focusing lens power supply 13: Scanning power supply, 14: Objective lens power supply 15a, 15b, 15c: Switch 16: Operation terminal 17: Switch control circuit 18: Alignment correction circuit 19: Alignment error correction circuit 20: Constant excitation current Generator 21: Adder circuit

Claims (1)

(57) [Claims] An image obtained by stopping a scanning current flowing through a scanning coil, superimposing the scanning current on an alignment coil, and scanning an electron beam emitted from an electron gun on an objective lens diaphragm to a cathode ray tube. What is claimed is: 1. An alignment apparatus for a scanning electron microscope which performs display and alignment correction, comprising: an alignment correction means for supplying said scanning current superimposed on a direct current for alignment; and an electron beam scanning by said alignment coil for scanning of said cathode ray tube. A correcting means for switching the supply of the scanning current to the alignment coil in order to correct an error in the alignment correction based on the phase delay of the alignment.