JPH0234139B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for aligning an electron optical lens barrel that allows simple and highly accurate alignment.

Conventionally, when aligning the beam axis of an electron optical column with a variable beam shape, a crossover reduced image of the electron beam is formed, and this beam scans the sample surface to detect the sample image and display the image on a CRT. This is done by adjusting the alignment coil so that only the so-called blur of the image changes when the excitation of the electron lens is minutely changed, and then adjusting the strength of the regular shaped beam electron lens. There is. As another method, a sensor such as a Faraday cup or a semiconductor detector is placed in the middle of the lens barrel, and alignment adjustment is performed using information on axis deviation obtained from this sensor. However, in the former method, when the strength of the lens is changed, the axial center position of the lens changes somewhat, so even if the axis is aligned using the crossover reduction image as described above, the axis often becomes misaligned when the beam is subsequently converted to a shaped beam. It was. Also, for the latter method,
Since a component (sensor) supported by an insulator is placed inside the lens barrel, the beam becomes unstable due to increased gas emission and charging, and therefore effective alignment cannot be expected.

The present invention has been made in consideration of these circumstances, and its purpose is to provide a practical method for easily and efficiently aligning the axes of all lenses of an electron optical lens barrel for a variable beam beam. The object of the present invention is to provide a method for aligning the axis of an electron optical lens barrel.

The present invention provides an axis alignment method for an electron optical lens barrel including a plurality of electron lenses and one or more beam shaping aperture masks in the electron beam path.
When a predetermined aperture is scanned with an electron beam, a target consisting of fine particles with high electron reflectance attached to a low electron reflectance substrate provided on the sample surface,
Axis alignment is performed by displaying an aperture image obtained from a so-called fine particle target and variably adjusting the alignment deflection means so that the position of the displayed aperture image does not change with changes in the amount of excitation of the electron lens. It is characterized by

According to the present invention, efficiency can be improved by a simple procedure of variably adjusting the alignment deflection means so that the display position of the aperture image obtained from the fine particle target does not change in response to changes in the excitation amount of the electron lens. axis alignment can be performed.

Further, in the present invention, since a fine particle target is used to obtain an aperture image required especially during alignment, the electron beam during alignment may be a shaped beam with variable dimensions as in actual use. For this reason, when returning to a shaped beam, the conventional method involves focusing the electron beam to form a crossover reduced image during alignment, and returning it to a regular shaped beam by greatly changing the excitation amount of the lens after alignment. The problem of misalignment is solved. Furthermore, there is no problem of beam instability caused by the method of arranging a sensor for obtaining information on axis deviation inside the lens barrel.

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

FIG. 1 is a diagram showing a schematic configuration of an electron optical lens barrel used for axis alignment, and 1 is an electron gun. The electron beam output from this electron gun 1 is aligned with the alignment coil 2.
is incident on the center of the condenser lens 3 by
The beam is focused by the same lens 3 and illuminated onto the first beam shaping aperture 4 . The aperture image of the electron beam formed by this aperture 4 is projected onto a second beam shaping aperture 6 by an electron lens 5, and at this time, the electron beam passes through the center position of the electron lens 5 under the control of an alignment coil 7. . Thereafter, the electron beam passing through the center position of the electron lens 5 is incident on the center of the reduction lens 9 under the control of the alignment coil 8, and then the electron beam passes through the center position of the alignment coil 1.
0 and enters the center position of the objective lens aperture 11. However, the reduction lens 9
As a result, the aperture image of the electron beam by the second beam shaping aperture 6 is transmitted to the objective lens aperture 1 via the two-stage deflection system 12a, 12b.
1 and is irradiated onto the sample surface 13 through the same lens aperture 11. The objective lens aperture 11 is controlled to reduce beam aberration.

Further, these lenses 3, 5, 9, objective lenses, and alignment coils 2, 7, 8, 10 are controlled by the CPU 15 via the interface 14, and are each excited and driven under predetermined conditions. In addition, the deflection systems 12a and 12b are composed of electrostatic deflection plates arranged to face each other in the X direction and the Y direction, which constitute a three-dimensional space, for example, when the axial direction of the electron beam is the Z axis. The deflection of the electron beam is controlled in response to a deflection scanning signal from a circuit 16. The reflected signal of the electron beam illuminated on the sample surface 13 is detected by a backscattered electron detector 17 made of a photomultiplier or the like, and this detection signal is inputted to a CRT monitor 19 via an amplifier 18. When adjusting the alignment, use this CRT monitor 19.
As a result, a reflected image is displayed in synchronization with the scanning of the electron beam in the deflection systems 12a and 12b.

Note that a means for suppressing beam aberration by the objective lens aperture 11 may be provided near the reduction lens 9, and the CPU 15 may also control the alignment coils 2, 7, 8, 10, lenses 3, 5, 9 and the objective. In some cases, a predetermined current is controlled and applied to each lens.

By the way, in an electron optical column having such a configuration, when scanning the sample surface 13 with a variable-dimensional beam-controlled electron beam, that is, a shaped beam, a two-stage deflection system (electrostatic deflection plate) is used. This is accomplished by applying deflection voltages with different polarities to 12a and 12b, and deflecting an electron beam deflected in one direction in the opposite direction to perform deflection without causing beam aberration, that is, beam shift. Therefore, if deflection voltages of the same polarity are applied to the deflection systems 12a and 12b, or if only one of them is driven, the electron beam will not be deflected in two stages and will follow the path shown by the broken line in the figure. In other words, the condition of passing through the center of the objective lens aperture 11 is no longer satisfied. For this reason, if the deflection voltages of the same polarity are applied to the deflection systems 12a and 12b and this is varied, the electron beam will be illuminated and scanned over the objective lens aperture 11, and in this two-dimensional scanning, it will happen that the aperture area The electron beam will reach the sample surface 13 only when the scanning position is located at . Therefore, if the electron beam transmitted through this aperture is detected from its reflected signal and displayed on the CRT monitor 19 in synchronization with the deflection scanning of the electron beam, an image corresponding to the aperture shape will be formed on the monitor 19. be able to. In this case, if a so-called fine particle target, in which fine particles with high electron reflectance are adhered to a substrate with low electron reflectance, is provided on the sample surface 13,
A good aperture image can be obtained while using a shaped electron beam.

Therefore, if the excitation amount of the condenser lens 3 is slightly changed, as long as the electron beam passes through the center position of the lens 3, the beam axis will not change due to the small displacement of the focal position. Second
As shown in Figure a, the aperture image position displayed on the CRT monitor 19 does not change; rather, as shown in Figure b, the image brightness, size, image blur, and resolution only change. Become. If there is a misalignment between the beam axis and the center position of the lens 3, the focal position will change slightly due to the minute change in the excitation amount, and this will change the curvature of the beam axis. This change is sequentially reflected in the lenses 5 and 9 of the next stage and the objective lens, and the image position changes as shown in FIG. 2a. Therefore, if the alignment coil 2 that controls the incident position of the electron beam on the condenser lens 3 is variably adjusted so that the image position is constant regardless of minute changes in the excitation of the lens 3, Axis alignment is performed effectively.

Thereafter, in the same manner, the axis of the lens 5 is aligned using the alignment coil 7, then the axis of the lens 9 is aligned using the coil 8, and finally the alignment coil 10 is adjusted to adjust the objective lens aperture 11.
By performing axis alignment for all lenses, it becomes possible to efficiently and highly accurately align all lenses.
It goes without saying that when aligning the other lenses 5, 9, and 11, the excitation may be slightly changed. Further, this alignment is performed in order from the lens on the electron gun 1 side. If this were reversed, it would be extremely inconvenient because each time the front and rear lenses are aligned, it would be necessary to repeatedly align the rear lenses.

As described above, it is possible to perform simple and highly accurate alignment by simply adjusting the alignment coil so that the position of the aperture image does not change when the excitation of the lens is slightly changed. Very expensive. In addition, in this case, there is no risk of instability of the electron beam as in the conventional case, and there is no possibility that the axis will be deviated during beam shaping. In other words, the control of the electron beam deflection system can be changed only when adjusting the alignment outside the optical lens barrel, and then returned to its original state, so there is no need to make any changes to the electron optical lens barrel itself, making it highly practical. . This effect is particularly noticeable when the lens has a multi-stage lens configuration, and the criteria for determining axis alignment are simple, making it easy to handle.

Note that the present invention is not limited to the above embodiments. For example, it is possible to use the alignment coil 10 instead of the deflection systems 12a and 12b as a means for scanning over the aperture, but it is still preferable to use an electrostatic deflection system because of the slow response speed. In addition, in the case of a structure in which the aperture provided in the objective lens is provided in the reduction lens 9, the aperture on the reduction lens 9 may be scanned with the electron beam, and in short, the aperture closest to the sample surface is scanned. All you have to do is scan. Also
Instead of displaying an image on the CRT monitor 19 and monitoring it, the scanning information and the detection signal from the backscattered electron detector 17 are input to the CPU 15, and automatic alignment control is performed so that the scanning voltage is always constant when the detection signal is detected. Of course, it is also possible to tighten it.
In short, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an example of an electron optical lens barrel subjected to axis alignment by the method of the present invention, Fig. 2 a, b
FIG. 2 is a diagram showing a change in image position and a change in image size for explaining the axis alignment principle of the present invention. 1... Electron gun, 2, 7, 8, 10... Axis alignment coil, 3, 5, 9... Lens, 11... Objective lens aperture, 12a, 12b... Deflection system, 1
3... Sample surface, 16... Scanning circuit, 17... Backscattered electron detector, 19... CRT monitor.

Claims (1)

[Claims] 1. In an alignment method for an electron optical lens barrel having a plurality of electron lenses and one or more beam shaping aperture masks in the electron beam path, a predetermined aperture is scanned with an electron beam. At this time, an aperture image obtained from a target formed by adhering high electron reflectance fine particles on a low electron reflectance substrate provided on the sample surface is displayed, and the position of the displayed aperture image is determined by the amount of excitation of the electron lens. A method for aligning an electron optical lens barrel, characterized in that the alignment is performed by variably adjusting an alignment deflection means so as not to change with respect to changes. 2. The method of aligning an electron optical lens barrel according to claim 1, wherein the predetermined aperture is an aperture provided at a position closest to the sample surface.