JPS6359224B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel magnetic field lens capable of correcting the difference between the voltage axis and the current axis.

In addition to the geometrical axis, the axes in the lens system of an electron microscope include a voltage axis and a current axis, and there is no argument that the latter two axes are more important. What is the voltage axis mentioned above? When the acceleration voltage is changed periodically while the strength of the lens system is kept constant, the image on the fluorescent screen expands and contracts radially around a certain point Oa, as shown in Figure 1. The current axis refers to the center point (an immovable point) when the acceleration voltage is kept constant and the lens strength (magnetic field strength) of the objective lens (lens system) is changed, as shown in Figure 2. As shown in the figure, this is the center point (unmoving point) when an image expands and contracts with rotation around a certain point Ob. If these voltage axis Oa and current axis Ob do not coincide with the center O of the fluorescent screen, the image at the center of the fluorescent screen will fluctuate as the accelerating voltage changes or the lens strength changes, resulting in blurring due to chromatic aberration. Therefore, high resolution cannot be obtained. Therefore, these axes Oa,
By aligning Ob with the center O of the phosphor screen using mechanical or electromagnetic alignment means, a portion free or less blurred due to chromatic aberration can be displayed on the phosphor screen.

However, the voltage axis and current axis are generally out of sync,
It was considered impossible to correct this disparity. For this reason, conventionally, as shown in FIG. 3, only one axis, for example, the current axis Ob, is aligned, but the other axis (voltage axis) is not aligned, so blurring of the image due to this cannot be prevented.

An object of the present invention is to provide a completely new magnetic lens capable of correcting the deviation l of the voltage axis and current axis that causes image blurring as described above. The structure of the present invention is characterized by a lens including an inner yoke cylinder, an outer yoke cylinder magnetically connected to the inner yoke cylinder, an excitation coil placed in a space surrounded by both yokes, and a magnetic pole part. A ferromagnetic auxiliary yoke member is installed outside the yoke inner cylinder in a rotationally symmetrical or axially symmetrical arrangement and has a length in the axial direction of the yoke inner cylinder, and the auxiliary yoke member is aligned with respect to the axis of the yoke inner cylinder. It consists of a magnetic field lens equipped with a mechanism for finely moving it within a plane at right angles to the magnetic field.

First, considering the cause of the difference in distance between the two axes, it can be said that it is primarily due to the hysteresis of iron. In other words, in a magnetic field lens, the magnetic flux generated by an excitation coil is passed through an iron yoke and collected in a gap by a magnetic pole piece made of iron, but changes in the current (magnetic flux density) flowing through the excitation coil and the magnetic field strength are due to hysteresis. It doesn't change linearly. If this hysteresis were completely axially symmetrical, it would not be a problem, but in general, it is not completely axially symmetrical due to variations in material composition, particles, or structural (processing) asymmetry, and this is the case when both axes are completely symmetrical. In other words, this causes a difference in distance l.

One way to reduce the effect of hysteresis is to
Although it is possible to reduce the magnetic flux density, such a method is completely impractical because the objective lenses of today's electron microscopes require excitation strong enough to cause magnetic saturation.

Therefore, the present invention corrects the shaft spacing difference due to imperfection of hysteresis in a strongly excited state. By controlling the flow of magnetic flux in the yoke, the current axis can be freely adjusted, thereby making it coincide with the voltage axis. I did it so that I could get it.

FIG. 4 schematically shows the flow of magnetic flux within the lens. In the figure, 1 is an inner yoke cylinder, and 2 is a U-shaped yoke outer cylinder, both of which are magnetically coupled at one end (the lower end of the inner cylinder 1), and the upper end of the yoke inner cylinder 1. A magnetic pole gap 3 is formed between the outer cylinder 2 and the upper part thereof. Note that a magnetic pole piece is usually inserted into this portion. Reference numeral 4 denotes an excitation coil wound in a space formed by the inner and outer yoke cylinders, and is connected to an external DC power source.

In such a lens, when an excitation current is passed through the excitation coil 4, a magnetic flux is generated as shown in the figure, passing inside and outside the yoke, but in the yoke part it forms a circle A.
Concentration of magnetic flux can be seen in the area surrounded by , that is, the area where the yoke inner tube 1 and the outer tube 2 are connected and which is in contact with the coil 4. Therefore, by adjusting the flow of magnetic flux in this portion, it is possible to efficiently change the magnetic field distribution within the magnetic pole gap 3.

One embodiment of the invention is shown in FIG. In this figure, the same reference numerals as in FIG. 4 indicate the same components, and 5 is a magnetic pole piece provided in the magnetic pole gap 3. A cylindrical ferromagnetic auxiliary yoke 6 is placed between the yoke inner tube 1 and the excitation coil 4, and is movable in a plane perpendicular to the lens axis (the axis of the yoke inner tube). This auxiliary yoke has several adjustment screws 7
The tips of the lenses a, 7b, . The auxiliary yoke is preferably a cylindrical body having a length in the direction of the lens axis O as shown in the figure, but it may also be a body formed by integrating a large number of rods axially symmetrically.

In such a structure, if the position of the auxiliary yoke is adjusted by, for example, pushing in the screw 7a and pulling out the screw 7b, the left side approaches the yoke inner cylinder 1, and the right side approaches the yoke inner cylinder 1, as shown in FIG. Cylinder 1
Becomes to move away from. For this reason, the yoke inner cylinder 1
A change occurs in the flow of magnetic flux at the junction between the yoke inner cylinder 1 and the outer cylinder 2, and as shown in FIG. 6, the number of magnetic fluxes passing through the yoke inner cylinder 1 is greater on the left side than on the right side. In other words,
This means that the distribution of magnetic flux density B can be adjusted. As a result, the magnetic field distribution within the magnetic gap in the pole piece 5 is affected, and its axis, that is, the current axis, moves. Therefore, the plurality of screws 7a, 7
By appropriately adjusting b..., it is possible to make the current axis coincide with the voltage axis.

As explained in detail above, the present invention arranges an auxiliary yoke made of a ferromagnetic material that is rotationally symmetrical or axially symmetrical and has a length in the axial direction of the yoke inner cylinder on the outside of the yoke inner cylinder,
This is made to move slightly in a plane perpendicular to the axis of the yoke inner cylinder, thereby adjusting the density distribution of the magnetic flux flowing inside the yoke inner cylinder, thereby moving the current axis. It becomes possible to correct the difference l between the voltage axis and the current axis, and it is possible to prevent a decrease in resolution due to a shift between both ends. In addition, the present invention adjusts the flow of magnetic flux in section A (Fig. 4) where the magnetic flux density is high, and since no magnetic material is inserted into the electron beam path, the generation of deflection fields and astigmatism can be avoided. Initial objectives can be achieved with little contribution.

Incidentally, although the above description has mainly been given to the objective lens of an electron microscope, it goes without saying that the present invention can also be used for other lenses.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the voltage axis, Figure 2 is a diagram for explaining the current axis, Figure 3 is a diagram for explaining the state of the axis in a conventional electron microscope, and Figure 4 is a diagram for explaining the axis of the present invention. FIG. 5 is a cross-sectional view showing an embodiment of the present invention, and FIG. 6 is a diagram explaining the operation of the present invention. 1: Yoke inner cylinder, 2: Yoke outer cylinder, 3: Magnetic pole gap, 4: Excitation coil, 5: Magnetic pole piece, 6: Ferromagnetic auxiliary yoke, 7a, 7b...: Adjustment screw.

Claims (1)

[Claims] 1. A lens comprising a yoke inner cylinder, a yoke outer cylinder magnetically connected to the yoke inner cylinder, an excitation coil placed in a space surrounded by both yokes, and a magnetic pole part, A ferromagnetic auxiliary yoke member is arranged outside the yoke inner cylinder in a rotationally symmetrical or axially symmetrical arrangement and has a length in the axial direction of the yoke inner cylinder, and the auxiliary yoke member is arranged at right angles to the axis of the yoke inner cylinder. A magnetic field lens characterized by having a mechanism for finely moving it within a flat plane. 2. The magnetic field lens according to claim 1, wherein the auxiliary yoke member has a cylindrical shape.