JPS6120108B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microscopic image photographing device for an electron microscope that is capable of photographing a sample while minimizing damage caused by electron beam irradiation during focusing and astigmatism correction.

In general, as a preparatory step for recording information obtained from an electron microscope, confirmation items such as axis alignment, finding the target field of view, astigmatism correction, and focusing are unavoidable. In this case, if the sample is damaged by electron beam irradiation during such preparatory operations, the original properties of the sample will change and the objective cannot be achieved. However, at observation magnifications of 10,000 times or more, the preparatory steps mentioned above, especially during astigmatism correction and focusing, are performed in a state in which the irradiated electron beam is narrowed down to 1 μm or less by the final focusing lens on the sample (that is, in a focused state). ), the electron current density is high and the sample is susceptible to electron beam irradiation damage.

Therefore, in order to solve these conventional drawbacks and to enable each step from searching the field of view to completing photography to be carried out efficiently, the earlier application, Japanese Patent Publication No. 1983-19408, proposed three operation modes. In the first operation mode, the excitation of the focusing lens for sample irradiation is set to a pre-stored value suitable for searching the field of view, and in the second operation mode, the excitation of the focusing lens is set to a pre-stored value suitable for searching the field of view. The deflection device is set to a value suitable for astigmatism correction and focusing, and the deflection device is controlled so that the sample irradiation position of the electron beam is shifted by a certain amount. In the third operation mode, the excitation of the focusing lens is stored in advance. The value is set to a value suitable for the photograph taken, and the photograph is taken by the photographing device.

However, in such conventional devices, when focusing, the irradiated electron beam is irradiated with high density at a position shifted a certain distance from the target field of view (field of view on the optical axis) on the sample, thereby Because the inflow of heat is extremely biased to a localized area that is off the optical axis, when we immediately switch to imaging mode, that is, when we move the center of the irradiated electron beam onto the optical axis, the sample undergoes a thermal drift (transition). This method has the disadvantage that high-resolution photographs cannot be obtained.

The present invention aims to solve such inconveniences, and will be explained in detail below with reference to the drawings.

FIG. 1 is a schematic diagram showing an embodiment of the present invention, and numeral 1 indicates an electron gun. The electron beam EB generated by the electron gun 1 is focused by the first and second focusing lenses 2 and 3 and irradiated onto the sample 4. The electron beam transmitted through the sample 4 by the electron beam irradiation is enlarged and imaged by a magnifying lens system consisting of objective, intermediate, and projection lenses 5, 6, and 7, and an enlarged image of the sample is projected onto the fluorescent plate 8. Ru. Further, an enlarged image of the sample is photographed by a photographing device 10 when the fluorescent plate 8 is opened by the fluorescent plate drive mechanism 9. Reference numeral 11 denotes a shutter plate used for photographing, which is opened and closed by a shutter drive mechanism 12. 13 is the second
This lens control circuit supplies excitation current to the focusing lens according to the voltage values from the three reference power supplies 15a, 15b, and 15c via the changeover switch 14. 16 is a switching circuit for switching the switching switch 14;
It is controlled by push button switches 17a, 17b and 17c provided corresponding to the respective terminals a, b and c of 4. 18 is a horizontal movement mechanism for horizontally moving the sample 4 mechanically. Reference numeral 19 denotes a lens control circuit for controlling the excitation currents of the objective, intermediate and projection lenses 5, 6, and 7.

Denoted at 20a and 20b are two-stage deflection coils (the number of deflection coils may be one-stage) placed between the second focusing lens 3 and the sample 4, and are controlled by a control circuit 21. The control circuit 21 receives command signals generated by pressing (turning on) the pushbutton switches 17a, 17b, and 17c, and also receives command signals from the first delay circuit 2.
A command signal from the push button switch 17c via 2 is supplied to each of the push button switches 17c. The control circuit 21 supplies preset constant deflection signals to the deflection coils 20a+20b based on command signals from the pushbutton switches 17a and 17b, respectively, so that the irradiated electron beam
It works to shift the center of the EB a certain distance from the optical axis. In addition, based on the command signal from the push button switch 17c, the control circuit 21 supplies a blanking signal to the deflection coil 20a to largely deflect the irradiated electron beam EB out of the electron beam path so that it does not reach the sample 4. Further, when the command signal from the push button switch 17c is supplied after a delay of a set time (for example, about 10 seconds) by the first delay circuit 22, the control circuit 21 changes the blanking signal supplied to the deflection coil 20a. unlock.

The output signal from the first delay circuit 22 is
The signal is supplied to the shutter drive mechanism 12 via the delay circuit 23, and the shutter plate 11, which has been closed, is opened based on the command signal.

Reference numeral 24 denotes an exposure control circuit of the photographing device 10, and the exposure control circuit is driven by the output signal from the second delay circuit 23.

In such a device, the electron beam EB from the electron gun 1
The electron beams are narrowly focused by the first and second focusing lenses 2 and 3 and irradiated onto the sample 4, and by controlling the second focusing lens 3, the density of the irradiated electrons on the sample 4 is controlled. The voltage value of the reference power source 15a in the second focusing lens 3 is set to supply the second focusing lens 3 with a current value suitable for searching the field of view, and the voltage value of the reference power source 15b is set to supply the second focusing lens 3 with a current value suitable for searching the field of view. The voltage value of the reference power source 15c is set to supply the second focusing lens 3 with a current value suitable for astigmatism correction, and a current value suitable for photographing.

Now, when the push button switch 17a is turned on, the switching circuit 16 connects the switching switch 14 to the terminal a side, so that the second focusing lens 3 is supplied with a current value suitable for visual field searching. That is, as the optical diagram is shown in Fig. 2a, the second focusing lens 3 is strongly excited, so the irradiated electron beam EB is focused above the sample 4, and a wide area of the sample is irradiated. , the electron current density on sample 4 becomes low. Also, since the command signal from the push button switch 17a is supplied to the switching circuit 16 and at the same time to the control circuit 21, the center of the irradiated electron beam is aligned with the optical axis by the deflection coils 20a and 20b as shown in FIG. 2a. The electron beam is deflected outward so that a portion of the edge portion of the irradiated electron beam is located on the optical axis. In this state, while observing the sample image projected onto the fluorescent plate 8 (in this case, the sample image is projected to the right with the optical axis on the fluorescent plate as the center), the horizontal movement mechanism 18 moves the sample 4 to the first position. By moving to the left in FIG. 2a, the desired field of view point A is moved to the center of the fluorescent plate 8, that is, the field of view point A is positioned on the optical axis.

After the field of view search is completed, when the push button switch 17b is turned on, the switching circuit 16 connects the switching switch 14 to the terminal b side as shown by the solid line in FIG. is supplied with a current value suitable for focusing and astigmatism correction. As a result, the excitation of the second focusing lens 3 is weakened, as shown by the solid line in FIG. Density increases. At this time, when the push button switch 17b is turned on, the changeover switch 14 is switched from the terminal a side to the terminal b side, and at the same time, the command signal from the push button switch 17b is also supplied to the control circuit 21, so that the control circuit 21 controls the deflection coil 20a, A constant deflection signal is supplied to 20b. Therefore, the irradiated electron beam EB is the second
B in sample 4 as shown by the solid line in Figure b.
The electron beam that irradiates a point and passes through point B is the object,
Fluorescent plate 8 by means of intermediate and projection lenses 5, 6, 7
An enlarged image is formed at the upper position P.

If the distance from point A to point B to be photographed in sample 4 is short, for example within 3 μm, it has been confirmed that the deflection aberration and the off-axis aberration of the objective lens are sufficiently smaller than the axial aberration. Therefore, the focusing and astigmatism correction performed at point B can be applied to point A as they are. Therefore, as shown by the solid line in Figure 2b, point B in the sample 4 was irradiated with a narrowly focused electron beam EB,
Focusing and astigmatism correction are performed while observing a sufficiently bright sample image above.

After completion of such operations such as focusing, the push button switch 17c is turned on to generate the command signal shown in FIG. Since a current suitable for photographing is supplied to the second focusing lens 3, and at the same time, a command signal is supplied to the control circuit 21, the control circuit 21 connects the deflection coil 20a to the direction b in FIG. A blanking signal shown in is supplied, and the irradiated electron beam EB is deflected out of the electron beam path as shown by the dotted line in the center of Figure 2b (this direction of deflection corresponds to the direction in which the irradiated electron beam is deflected when searching for a field of view or focusing. ) and do not allow it to reach sample 4. Furthermore, since the command signal is simultaneously supplied to the fluorescent plate drive mechanism 9 and the shutter drive mechanism 12, each drive mechanism 9 and 12 is driven as shown in FIGS. 3c and 3d, and the fluorescent plate 8 is opened. Also, the shutter plate 11 is closed. Furthermore, since the command signal from the push button switch 17c is simultaneously supplied to the first delay circuit ₂₂ , the output signal shown in FIG. The blanking signal sent to the circuit 21 and supplied to the deflection coil 20a is canceled, and the center of the irradiated electron beam EB is positioned on the optical axis. At this time, since the second focusing lens 3 is connected to the reference power source 15c suitable for photographing, the excitation is strengthened.
The irradiation electron beam EB has a large spot diameter as shown in FIG. 3a, and irradiates a wide area of the sample 4. Since the output signal from the first delay circuit 22 is further sent to the second delay circuit 23, the second
As shown in Fig. 3f, the delay circuit ₂₃ of
(For example, after about 0.1 seconds), an output signal is supplied to the shutter drive mechanism 12 and the exposure control circuit 24. As a result, the drive of the shutter drive mechanism 12 is stopped and the shutter plate 11 is stopped as shown in FIG.
is opened, so that an enlarged image of point A in the sample 4 is exposed to the film of the photographing device 10. However, when the predetermined exposure time _T3 has elapsed, the exposure control circuit 24
Since the output signal is supplied to the shutter drive mechanism 12 from the shutter plate 11, the shutter plate 11 is closed. The exposure control circuit 24 also sends an output signal to the photographing device 10 to store the photographed film in the storage box and set the unphotographed film at the photographing position. Further, the exposure control circuit 24 supplies a reset signal shown in FIG. This allows the third
As shown in FIG. c, the driving of the fluorescent plate drive mechanism 9 is stopped and the fluorescent plate 8 is closed. Also changeover switch 1
4 is connected to the terminal b side as shown by the solid line in FIG. 1, and the irradiation electron beam EB is set so as to irradiate a preset point B in the sample 4. Further, the first and second delay circuits 22 and 23 are arranged as shown in FIG.
They are reset as shown respectively in . The shutter plate 11 is configured to open when the fluorescent plate 8 is closed. Therefore, if the focal length of the objective lens 5 is arbitrarily changed in this state,
Sample images with different focus states can be taken.

In this embodiment, since the position of the focusing spot image (that is, the image corresponding to point B in the sample 4) changes as the magnification changes, the control circuit 21 is activated in conjunction with the operation of the lens control circuit 19. It is necessary to control the amount of deflection in the deflection coils 20a and 20b so that the image is always focused on the P position on the fluorescent plate 8.

It should be noted that the above description is an illustration of the present invention, and many modifications may be made in implementing the present invention.

For example, we have described the case where a deflection coil used for field searching, focusing, etc. is also used as a deflection coil for blanking the irradiated electron beam. You can also prepare a board.

Further, although it has been described that the center of the irradiated electron beam is shifted from the optical axis when searching the field of view, it is not necessarily necessary to shift it.

Furthermore, although it has been described that the image of point B in the sample for focusing is formed at a certain P position outside the center of the phosphor plate, the image is not limited to this, but may be formed between the objective lens and the intermediate lens. By installing two stages of deflection coils (one stage of deflection coils may be used if this deflection coil is installed at or near the back focal plane of the objective lens), the image of point B in the sample can be set at the center of the fluorescent plate. It is also possible to form an image on the area. In this case, the deflection direction of the deflection coil placed between the second focusing lens and the sample, and the deflection coil placed between the objective lens and the intermediate lens may cause distortion due to rotation of the image caused by changes in magnification, etc. , it is necessary to control in connection with the operation of the lens control circuit of the imaging lens system.

As is clear from the above description, in the present invention, when the third operation mode is started in order to take a photograph, the control means excite the focusing lens for electron beam irradiation in a pre-stored manner suitable for taking a photograph. At the same time, the blanking means blanks the electron beam for a certain period of time, and after canceling the blanking, shooting starts. Even if a high-density electron beam is irradiated to a sample position that is off-axis, photographs can be taken after the thermal drift caused by the heat inflow being extremely localized to a localized area off-axis has been alleviated. Therefore, high-resolution photography can be reliably performed without any operational errors by simply starting the third mode.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of the device of the present invention, FIGS. 2a, b, and c are optical diagrams for explaining the operation of the embodiment of the device shown in FIG. 1, and FIG.
Figures a to h are explanatory views of the operation of the embodiment of the apparatus shown in Figure 1. DESCRIPTION OF SYMBOLS 1... Electron gun, 2 and 3... First and second focusing lenses, 4... Sample, 5, 6, and 7... Objective, intermediate and projection lenses, 8... Fluorescent plate, 9... Fluorescent plate drive mechanism, 10... Photographing device, 11...Shutter board, 12
... Shutter drive mechanism, 13 and 19... Lens control circuit, 14... Changeover switch, 15a to 15c
...Reference power supply, 16...Switching circuit, 17a to 17c
...Push button switch, 18...Horizontal movement mechanism, 20a and 20b...Deflection coil, 21...Control circuit, 22 and 23...First and second delay circuit, 24...Exposure control circuit.

Claims (1)

[Claims] 1. A focusing lens for focusing the electron beam generated from the electron gun and irradiating it onto the sample, and a blanking lens for preventing the electron beam from irradiating the sample by deflecting the electron beam. a deflection means for moving an electron beam irradiation position on the sample, and a photographing means for photographing an electron microscope image, a control means having three operation modes is provided, In a first mode of operation by the control means, the excitation of the focusing lens is set to a pre-stored value corresponding to low-intensity irradiation suitable for field searching, and in a second mode of operation, the excitation of the focusing lens is set to an astigmatic value. The deflection device is set to a pre-stored value corresponding to high-density irradiation suitable for correction and focusing, and the deflection device is controlled so that the sample irradiation position of the electron beam is shifted by a certain amount. The excitation of the focusing lens is set to a pre-stored value suitable for photographing, and the blanking means blanks the electron beam for a certain period of time, and after canceling the blanking, the photographing device performs photographing. 1. A microscopic image photographing device for an electron microscope, characterized in that: 2. A microscopic image photographing device for an electron microscope according to claim 1, wherein the blanking means is also used by the deflecting means.