JPS59158061A - Objective lens magnetic pole piece - Google Patents

Abstract

PURPOSE:To increase the spherical and the chromatic aberration coefficients of an objective in which a sample is inserted in lateral direction by specifying the dameters of the holes and the diameters of the top surfaces of an upper and a lower pole piece to be different under the practical conditions of the distance between the pole pieces, the position of the sample and the taper angles of the pole pieces. CONSTITUTION:A side-entry-type objective in which a sample 6 is inserted in lateral direction, is constituted by installing pole pieces 4 and 5 integrated with a spacer 3 above a yoke 1 in which an excitation coil 2 is installed. The distance (S) between the pole pieces 4 and 5 is adjusted to not less than 5mm., the sample 5 is located Z0 over the center of the distance (S), and the taper angle (theta1) of the pole piece 4 is made larger than the taper angle (theta2) of the pole piece 5. In addition, when the diameters of the holes of the pole pieces 4 and 5 are (b1) and (b2) and the diameters of their top surfaces are (D1) and (D2), they are adjusted to satisfy the formulas of D1<D2 and b1<=b2<S. As a result, the spherical and the chromatic aberration coefficients of the lens can be decreased without increasing the diameter of the top surface of the pole piece, thereby enabling image observation of high resolution to be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic pole piece of an objective lens used in an electron microscope or the like.

Objective lens pole piece shape for high-resolution electron microscopy.

It has long been known to have a narrow gap and a small hole diameter, but when both the gap and the hole diameter are made small, it becomes difficult to insert the sample, which poses a practical problem. In order to solve this problem, a so-called asymmetric lens, in which the hole diameter of the upper magnetic pole piece is made larger than that of the lower magnetic pole piece, has been developed and is used in the top entry type sample insertion method. However, the method of inserting the sample is not limited to the above-mentioned top entry method; the side entry method, in which the sample is inserted from the side of the lens gap, is also widely used, and it is possible to insert a sample holder into the side entry. A lens with a wider gap is used. However, a lens with a wide gap has a higher spherical aberration coefficient O3 than a lens with a narrow gap.
It is extremely difficult to make 1-nons with a size of 2n1m or less, and therefore high-resolution electron microscope images cannot be obtained. Then,
A pinhole lens is one of the lenses that have been proposed to reduce the spherical aberration coefficient Cs.

In this lens, the hole diameter of the upper magnetic pole piece is made significantly smaller than that of the lower magnetic pole piece, but it is difficult to make the hole diameter of the upper magnetic pole piece even smaller, which has such a small hole diameter. There are many difficulties in processing, and it is not practical unless the overall dimension of the lens is originally large. Furthermore, since the magnetic shielding coil of this lens is divided into upper and lower sections with respect to the sample position, it cannot be used in recent analytical electron microscopes that are equipped with an X-ray analyzer or the like.

In view of the above points, the present inventor has proposed a shape of a lens pole piece that can reduce the spherical aberration coefficient and chromatic aberration coefficient of a lens into which a sample is laterally inserted under various conditions. The lens will be explained below.

First, we examined the practical conditions for a practical objective lens in which an excitation coil exists only below the sample position, as shown in FIG. In the figure, 1 is a yoke of an objective lens, and an excitation coil 2 is housed inside the yoke.

Above the yoke, an upper magnetic pole piece 4 and a lower magnetic pole piece 5, which are integrated by a spacer 3, are magnetically connected.

Reference numeral 6 denotes a sample, which is inserted through a hole in the lens yoke from a direction perpendicular to the optical axis.

The first practical condition for such a lens is the distance S between the upper and lower magnetic pole pieces.
It is. The thickness of the sample holder 7 currently in use is 2
mm to 3 mm, and this is ±15 as shown in Figure 2.
If the sample holder is tilted at an angle of about .degree., and the aperture 8 is also inserted below the sample holder, the magnetic pole spacing S needs to be at least about 5 mm.

The second is the sample position Zo. In an electron microscope, an aperture must be inserted below the sample to provide contrast. Therefore, when inserting a sample within the magnetic pole spacing S secured under the first condition, the boulder holding the sample will be placed above the aperture space, and the sample position will inevitably be within the magnetic pole spacing. Somewhat above the center of (Zo
It will be placed in >o>.

The third is the taper angle (the angle at which the optical axis meets the optical axis on one side of both magnetic poles) θ
It is. The angle 01 of the upper pole piece vi1 is the angle θ2 of the lower pole piece
It has to be larger (θ1>θ2).

I don't get it. If θ1 and θ2 are equal, or if θ2 is larger than θ1, the total length of the objective lens will be longer than in the case where 01>θ2, making it impractical.

Under the above conditions, the present inventor empirically designed the best possible lens shape and investigated its performance. The dimension of each part of the lens is s=5 mm.

θ+=70", θ2=60', upper and lower!i magnetic pole piece top surface diameters D+ and D2 are equal to each other D=8 mm, hole diameters of upper and lower magnetic pole pieces b+ and bz are equal to each other, b=4m
This is the case where m is used. Diagram M3 shows the performance of the lens, where the horizontal axis is the sample position Zo (mm), and the vertical axis is the focal length f o (ml) and the spherical aberration coefficient Cs (+t+m).
, chromatic aberration coefficient @Cc (mllll). As can be seen from the internuclear space, when the sample position Zo is about Zo = 0.25 mm, f o = 2.1 mm, Cs = 1.3
mm, CG = 15 mm, indicating that the shape is sufficiently suitable for the magnetic pole piece.

In such a lens in which the diameter of the top surface of the magnetic poles and the diameter of the hole are equal to each other, a universal relationship between the hole diameter of both magnetic pole pieces and the diameter of the top surface of the magnetic pole piece with respect to the hole diameter was pursued. FIG. 4 shows the appropriate hole diameter, where the horizontal axis shows the hole diameter b of both magnetic pole pieces, and the vertical axis shows f, ), Cs, and Cc. Also, S, θ1.
θ2 is the same as above, the diameter of the top surface of the magnetic pole piece is Bmm, that is, when D/b = 2, and the acceleration voltage of the electron beam is 20
This is the case of 0KV. From the internuclear distance, the focal length 111'o and the chromatic aberration coefficient CC are smaller as b is smaller, b = 2.5 mm
There is a minimum value near (b = s / 2), and the spherical aberration coefficient O8 tends to become smaller as b becomes larger.
The minimum value is shown in the vicinity of 5 mm (b = s >). From this optical characteristic diagram, it was found that a hole diameter of about 2°5 ml1 to about 4 mm, that is, b: about 3 mm is practical.

On the other hand, regarding D/b, when the hole diameter is fixed at the optimal 3 mm, C
The values of s and Cc were obtained. From the internuclear side, the larger D/b is, the smaller C3 and CC are, and from around D/b = 2 @
It decreases drastically, and when D/b is around 5 to 6, C3 = 1.2 mm
, Cc = 1.4 mm, which was an extremely good result. Therefore, if D/b is made as large as possible when b is 3 mm, a high-resolution objective lens pole piece can be obtained.

However, as can be seen from Fig. 5 above, in order to reduce the spherical aberration CS and chromatic aberration CC, D/b must be increased. Under the condition of =6, D becomes as high as 18 mm, which causes a slight problem in that the diameter of the top surface of the magnetic pole piece becomes large and the yoke connected to it becomes large.

Therefore, the object of the present invention is to improve the lenses shown in FIGS. 3 to 5 above, and to provide a side entry type lens that can reduce the spherical aberration coefficient C3 and the chromatic aberration coefficient CC without increasing the diameter of the top surface of the magnetic pole piece too much. The object of the present invention is to provide a magnetic pole piece for an objective lens.

The configuration of the present invention is of a type in which the sample is inserted from a direction perpendicular to the electron beam optical axis, and the spacing S between the upper magnetic pole piece and the lower magnetic pole piece is 5 mm or more, and the sample is inserted between the upper magnetic pole piece and the lower magnetic pole piece. The taper angle θ of the upper magnetic pole piece is
1 is larger than the taper angle θ2 of the lower magnetic pole piece, the hole diameter of the upper magnetic pole piece is bl, and the hole diameter of the lower magnetic pole piece is b.
2, and the top surface diameter of the upper magnetic pole piece is Dl, and the top surface diameter of the lower magnetic pole piece is D2, then Dl < D2 , [1+ ≦ b2
It is characterized by the magnetic pole pieces of the objective lens, which are made into S.

The present invention will be explained below based on FIGS. 6 to 9.

The inventor of the present invention thought that it would be good if the hole diameter and top surface diameter of the upper and lower magnetic pole pieces were different from each other, and conducted various experiments. FIG. 6 shows the symbols of each part of the asymmetrical magnetic pole piece, where bl is the hole diameter of the upper magnetic pole piece, b2 is the hole diameter of the lower magnetic pole piece, Dl is the top surface diameter of the upper magnetic pole piece, and D2 is the lower magnetic pole piece. The diameter of the top surface of each is shown. Other symbols are the same as in FIG. First, b 1 = b 2 = 3 mm, D2, which seems to be the best
= fo according to the change in Dl when it is 12 mm,
Changes in O3 and CC were determined. FIG. 7 shows its optical characteristic diagram, in which Dl is plotted on the horizontal axis and fo, C3, and CG are plotted on the vertical axis. As can be seen from the internuclear space, the smaller Dl becomes, the more f
o, C3, (, C are all small, so it was found that it is better to make the top surface diameter of the upper magnetic pole piece as small as possible.

Further, FIG. 7 shows the relationship between the top surface diameter D1 of the upper magnetic pole piece and the top surface diameter D2 of the lower magnetic pole piece. In the internuclear space, the horizontal axis is Dl, the vertical axis is D2, and the spherical aberration coefficient O3 is used as a parameter. C31 is C5=1.25mm, C32 is C3=1.3II1m], then C8
3 is a case where C5=1.35 mm. From this figure, Dl
It can be seen that the smaller D2 is, the smaller the spherical aberration coefficient O3 becomes. It was also found that the region above the dotted line in the figure, that is, the region where Dl<D2, gives very good results, and furthermore, the region where 2Dl<D2 gives very good results.

Figure 9 shows the practically best Dl in Figure 8, which is 6mm.
This figure shows the changes in fo, Cs, and Cc when 5Dz is set to 12ml and bl and b2 are changed. The internuclear figure corresponds to 11'1.4 j'4, and the horizontal axis is b
2, vertical axis i, mouth 0. C3 and CC are shown respectively.

In the figure, if the solid line is =3n+m, the dotted line is bl-2m
In the case of m, obviously BL is small J) The smaller the lens v
I Bamboo IJ will get better. Therefore, it can be seen that it is difficult to make the relationship b1≦1)2. As for b2, as with the conclusion in FIG. 4, it is better that b2 is about 3 mm, and that b2 is smaller than the magnetic pole spacing S.

In obtaining the data in FIGS. 7 to 9, the dimensions and acceleration voltages of each part are the same as in FIGS. 3 to 5.

As explained above, in the present invention, the magnetic pole spacing S, the sample position Zo, and the magnetic pole taper angle θ1. Under practical conditions of θ2, the hole diameter and top surface diameter of the upper and lower magnetic pole pieces are made asymmetrical,
This provides an appropriate relationship between them, which allows for the installation of an X-ray analyzer, etc., without increasing the diameter of the top surface of the magnetic pole piece, and also provides a spherical aberration coefficient comparable to that of the conventional top entry type. C3 and chromatic aberration coefficient CC can be achieved,
Even with Side En 1 Edge 1 type, an objective lens pole piece capable of observing extremely high-resolution electron microscope images or 7 iJ can be obtained.

In addition, KIP 1! The data is for when the accelerating voltage is 200 KV, or even when the accelerating voltage is different, the tendency is almost the same (-7, and the above hole diameters, b2, top diameters D+, and D2 are mutually different. The universality of the relationship holds true.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of the objective lens which is the premise of the present invention, FIGS. 2 to 5 are diagrams explaining the lens shape developed prior to the present invention, and FIG. 6 is a diagram showing the asymmetric lens shape of the present invention. The figures showing the shape and FIGS. 7 to 9 are diagrams for explaining the present invention. 4: Upper magnetic pole piece 5: Lower magnetic pole piece 6; Sample 7: Sample holder 8: Aperture, Patent applicant JEOL Ltd. Representative Ito-O (Fig. 2) 1 Fig. 3-0500, S-N ZoC Materials) /134-3 11 → b (笥箮) Fig. 6-〉I) + (front tube) Fig. 8 2 4 6. 5/ρ→Q1 (t
n enemy) 323 → courtesan (tn chiku)

Claims (1)

[Claims] A type in which the sample is inserted from a direction perpendicular to the electron beam optical axis, the distance S between the upper magnetic pole piece and the lower magnetic pole piece is 5 mm or more, and the sample is placed above the center of the upper and lower magnetic pole gap. In the lens in which the taper angle θ1 of the upper magnetic pole piece is larger than the taper angle θ2 of the lower magnetic pole piece, the hole diameter of the upper magnetic pole piece is bl, the hole diameter of the lower magnetic pole piece is b2, and the top of the upper magnetic pole piece is A vi1 pole piece of an objective lens, characterized in that, where Dl is the surface diameter and D2 is the top diameter of the lower pole piece, Dl < D2 and b 1 < b2 < s.