JPS6012739B2 - Objective lenses for scanning electron microscopes, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens for a scanning electron microscope, etc., which allows a minimum spherical aberration coefficient Cs to be obtained with respect to changes in working distance.

Generally, in a scanning electron microscope, depending on the purpose of observation,
It is necessary to vary the working distance W and D of the objective lens. For example, when performing x-ray analysis,
Since the take-off angle of the line must be increased, the sample must be kept far away from the bottom surface of the objective lens, and the working distance must be increased.Also, in cases such as high-resolution dragon secondary electron image observation, The need to strongly excite the lens and create a small working distance means that
This is a well-known fact. By the way, in conventional objective lenses, the excitation extreme is varied while the gap between the upper and lower magnetic poles is fixed, and the focal length is adjusted in response to changes in the working distance, but according to the inventor's experiments, this is not the case. It has been found that in lenses, the working distance at which the spherical convergence coefficient Cs reaches a maximum of 4 is at one point, and in other areas Cs is used at the expense of the working distance. Accordingly, the present invention provides an objective lens for a scanning electron microscope, etc., which can obtain a minimum spherical aberration coefficient even when the working distance changes.

Figure 1 shows experimental data for explaining the principle of the present invention.
Changes in the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc with respect to changes in the working distance W·D are displayed using the distance S between the upper and lower magnetic poles as a parameter.

In this case, the hole diameters of the upper magnetic pole and the lower magnetic pole are constant for 3 sides and IQ cape, respectively, and the data is when the magnetic pole spacing S is varied to 4 sides, 1 side, and 16 sides. In the same figure, considering the spherical aberration coefficient Cs, in all cases, Cs decreases as the working distance W・D decreases, but at large working distances, as S increases, the spherical aberration coefficient Cs decreases. is relatively small. Furthermore, in the 4-square region of the working distance, the smaller S is, the smaller Cs is. On the other hand, considering the chromatic aberration coefficient Cc, as the overall working distance becomes smaller, C
Although c becomes smaller, there is almost no difference due to S at a large working distance, and a slight change due to S appears in an area where the working distance is small. In this case, the smaller S is, the more Cc is obtained. From the above considerations, in the region where the working distance is 4.0 cm, both Cs and Cc have smaller values as S becomes smaller. Therefore, the magnetic pole gap should be made as narrow as possible, and in the region where the working distance is large, Cc is almost equal to S. It was found that Cs is largely unrelated, and the larger S is, the more advantageous it is, so it is sufficient to increase the magnetic pole gap.

Although the experiment was performed with the magnetic pole spacing S outside the parameter, the phenomenon can be interpreted as a change in d during the half-maximum value of the Fujigami magnetic field within the magnetic pole gap.

Since there is a roughly proportional relationship between S and d, it can be safely assumed that the above experimental data excludes d in the half value. The present invention realizes the above principle,
FIG. 2 shows one embodiment.

In the figure, reference numeral 1 denotes an outer yoke of the objective lens, the lower part of which projects toward the sample, and the lower end of which constitutes the lower magnetic pole 2. Reference numeral 3 denotes an inner yoke, which has a cylindrical shape and is slidably inserted into the center of the outer yoke from above.

The lower part of this yoke constitutes the upper magnetic pole 4 and forms a magnetic gap with the lower magnetic pole 2. A rack 5 is carved into the upper side of the inner yoke, and a pinion 6 is engaged with it. The pinioso is driven by a drive mechanism 7. A sample 8 is placed below the lower magnetic pole 2 and is held on a stage 10 which can be moved up and down by a rack 9.

A pinion 11 is engaged with the rack, and the drive mechanism 1
2. The drive mechanisms 7 and 12 may be manual or electric using a pulse motor or the like. The two are interlocked or synchronized, and in association with the vertical movement of the stage 10, that is, the variation of the working distance W1D, the yoke 3 is moved up and down, and the magnetic pole spacing S is varied. That is, when the working distance of the sample position shown by the solid line is W.D, the corresponding optimum magnetic pole spacing is S, and since W.D. is large, S. is also getting bigger. Next, when the drive mechanism 12 is controlled to raise the stage 10 to the position shown by the dotted line, the working distance becomes smaller as shown by W·D2. In conjunction or synchronization with this, the drive mechanism 7 operates, the fork 3 descends, and the magnetic pole spacing is changed to the smallest value of spherical aberration and chromatic aberration, as shown in S2. Figure 3 shows
This is a sectional view showing a main part of another embodiment, and the difference is that the upper magnetic pole 4 is moved. That is, the inner yoke 3 is integrated with the outer yoke, the upper magnetic pole 4 is made slidable within the inner yoke, a rack 13 is formed on the upper outer periphery of the upper magnetic pole 4, and a pinion 14 driven from the outside is attached to the rack 13. It's a combination of things. FIGS. 4 and 5 show still another embodiment, in which the present invention is applied to a lens having two magnetic pole gaps S and S'.

Reference numeral 15 denotes an intermediate magnetic pole, which is fixed to the lower magnetic pole 2 by a non-magnetic material 16, and forms a constant gap S between it and the lower magnetic pole.

Reference numeral 17 denotes an auxiliary magnetic pole that is aligned with the upper magnetic pole 4 and is provided between the upper magnetic pole and the intermediate magnetic pole 15, and has a rack 18 on its outer periphery.
is formed and is moved up and down by the pinion 19.

As a result, the magnetic pole gap S' between the intermediate magnetic pole 15 and the auxiliary magnetic pole 17 can be arbitrarily varied from zero to a constant value. Fourth
The illustrated state is when the auxiliary magnetic pole 17 is fully moved toward the upper magnetic pole 4, and two gaps S and S' are formed. With such a magnetic pole structure, even if the sum of the values of S and S' is considerably smaller than the spacing in the case of a single gap, it is possible to widen the half-value of the axial magnetic field distribution. The state is valid when the working distance is large. Next, as shown in FIG. 5, when the pinion 19 is operated to lower the auxiliary magnetic pole 17 until it contacts the intermediate magnetic pole 15, the upper magnetic pole 4, the auxiliary magnetic pole 17, and the intermediate magnetic pole 15 become magnetically integrated, and the magnetic pole gap S ' disappears and becomes a single gap of only S. Therefore, for example, if the value of the gap S is set to 4 fences, when the working distance is 4.0, a maximum spherical aberration coefficient and chromatic aberration coefficient of 4.0 can be obtained, as can be seen from FIG. Incidentally, the gap S' can be arbitrarily adjusted to a value intermediate between those shown in FIGS. 4 and 5, and it goes without saying that the scale can be adjusted in conjunction with the working distance as shown in FIG. The embodiment shown in FIG. 6 is a modification of FIGS. 4 and 5,
There is a difference in the means for moving the auxiliary magnetic pole 17.

That is, while the embodiments shown in FIGS. 4 and 5 used mechanical means using a rack and pinion, this embodiment is characterized by using electromagnetic means using a solenoid. In the figure, solenoids 20 and 21 are embedded in an intermediate magnetic pole 15 and an auxiliary magnetic pole 17, respectively, and the auxiliary magnetic pole 17 is moved by controlling the current flowing through these solenoids. For example, when currents in different directions are applied to the solenoids 20 and 21, the auxiliary magnetic pole 17 is moved to the upper magnetic pole side as shown in the figure due to the repulsive force caused by the magnetic field. When they are excited in the same direction, the auxiliary magnetic pole 17 is drawn toward the intermediate magnetic pole 15 by the attractive force caused by the magnetic field. Note that the firing directions of the solenoids embedded in the respective magnetic fields 15 and 17 do not have to be as shown in FIG. A leaf spring may also be inserted between the auxiliary magnetic pole 17 and the yoke 4. As detailed above, in the present invention, when the working distance is changed, the magnetic pole gap is continuously or intermittently varied in relation to it, so that the spherical rhombic coefficient is changed for each working distance. can be minimized.

[Brief explanation of the drawing]

Fig. 1 is a data sheet for explaining the principle of the present invention, Fig. 2 is a sectional view showing an embodiment of the invention, Fig. 3 is a sectional view of main parts showing another embodiment, Figs. FIG. 5 is a diagram showing still another embodiment, and FIG. 6 is a sectional view showing a modification of FIG. 4. 1: Outer yoke, 2: Lower pole piece, 3: Inner yoke, 4
: Upper magnetic pole, 5 and 9: Rack, 6 and 11 three pinions, 7 and 12: Drive mechanism, 8: Sample, 10: Stage. Soup, Figure Soup Otsu Figure S Figure O 4 ■ Figure 5 O Figure 6

Claims (1)

[Claims] 1 In relation to changes in working distance, the half-width of the axial magnetic field distribution is increased for large working distances, and the gap between the upper and lower magnetic poles is adjusted to obtain a smaller half-width for small working distances. An objective lens for a scanning electron microscope, etc., characterized in that the objective lens is variable.