JPH01211841A - Objective lens for transmission electron microscope - Google Patents

Abstract

PURPOSE:To simultaneously obtain high resolution and high contrast by specifying the hole diameter of a lower magnetic pole piece in a lens maintaining the preset conditions for the diameter of the bottom face of an upper magnetic pole piece, the diameter of the top face of the lower magnetic pole piece, hole diameters of the upper and lower magnetic pole pieces, the distance between the upper and lower magnetic pole pieces, and the taper angle of the lower magnetic pole piece. CONSTITUTION:In an objective lens satisfying equations theta2<60 deg., 0.8S<=b1<=1.2S, D1<=3b, D2<=3b2, where D1 and D2 are the diameter of the bottom face of an upper magnetic pole piece 1 and the diameter of a lower magnetic pole piece 2 respectively, b1 and b2 are hole diameters of the upper and lower magnetic pole pieces respectively, S is the distance between the upper and lower magnetic pole pieces, theta2 is the taper angle of the lower magnetic pole piece, the hole diameter b2 of the lower magnetic pole piece is set to a range between 0.5S and S. The visual field of a sample image projected on a fluorescent plate is not cut even if an objective orifice is inserted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an objective lens used in a transmission electron microscope, and particularly to a shape of an objective lens pole piece that can simultaneously obtain high resolution and high contrast.

[Prior Art] Conventionally, as an objective lens for a transmission electron microscope that is capable of observing low-magnification images of biological sections and the like with good contrast, an objective lens having a structure disclosed in Japanese Patent Application Laid-Open No. 61-4143 has been known. In this objective lens, the diameter of the bottom surface of the upper magnetic pole piece and the diameter of the top surface of the lower magnetic pole piece are respectively DI, D2,
When the hole diameters of the upper and lower magnetic pole pieces are bl and b2, the interval between the upper and lower magnetic pole pieces is S, and the taper angle of the lower magnetic pole piece is θ2, θ2 is 6.
In the electronic lens set to 0°, 0.88≦bl≦1
It has been proposed to form the upper and lower magnetic pole pieces so as to satisfy the following conditions: 0.28, D+≧3b+, 0.3S<b2≦0,5S, and D2≦3b2.

[Problems to be Solved by the Invention] However, although the objective lens proposed above can provide good contrast, it is not necessarily sufficient in terms of resolution.

The present invention takes these points into consideration and aims to provide an objective lens that can obtain high resolution and high contrast at the same time.

[Means for Solving the Problems] The present invention provides that the diameter of the bottom surface of the upper magnetic pole piece and the diameter of the top surface of the lower magnetic pole piece are each D1. D2, the hole diameters of the upper and lower magnetic pole pieces are respectively b1. b2
, the spacing between the upper and lower magnetic pole pieces is S, and the taper angle of the lower magnetic pole piece is θ2.
and θ2 <60”, 0°88≦b1≦1.23.
When D≧3b1 and D2≦3, b2 is set to a value greater than 0.5S and smaller than S.

[Example] Hereinafter, an example of the present invention will be described based on the drawings. 1st
The figure is a configuration diagram of an objective lens for explaining one embodiment of the present invention. In FIG. 1, 1 is the upper magnetic pole piece, bl is the hole diameter of the upper magnetic pole piece 1, and Dl is the diameter of the bottom surface of the upper magnetic pole piece 1.

2 is the lower magnetic pole piece, b2 is the hole diameter of the lower magnetic pole piece 2, and D is the diameter of the top surface of the lower magnetic pole piece 2. S is the distance between the upper magnetic pole piece 1 and the lower magnetic pole piece 2, θ2 is the taper angle of the lower magnetic pole piece 2, and 3
4 is a spacer for supporting the magnetic pole piece, 5 is a yoke, 6 is an excitation coil, 7 is a sample, and 8 is an objective aperture.

The present inventor developed the above-mentioned Japanese Patent Laid-Open No. 61
Parameters of the proposed objective lens disclosed in Publication No. 4143, b1, b2. As a result of considering D1, D2°S, and θ2, other conditions (bl '+ DI r
It was found that b2 greatly contributes to the spherical aberration coefficient C8 and the longitudinal chromatic aberration coefficient Cc, which uniquely determine resolution, while maintaining D2 + Sr θ2).

Therefore, the structural conditions are that the taper angle θ2 of the lower magnetic pole piece 2 is θ2 × 60°, and the hole diameter of the upper magnetic pole piece 1 is 0.8S≦
b, ≦1.28, the diameter of the bottom of the upper magnetic pole piece 1 is D1≧3bl
Experimentally, an attempt was made to maintain the diameter of the top surface of the lower magnetic pole piece 2 at D2≦3b2 and vary the hole diameter b2 of the lower magnetic pole piece 2.

Figure 2 shows the hole diameter b2 of the lower pole piece 2 using the lens in Figure 1.
It is a diagram showing the relationship between b2, spherical aberration coefficient Cs, axial chromatic aberration coefficient Cc, and distance Zc between the sample and the back focal point when changing the diameter of the upper magnetic pole piece. The diameter D1 of the top surface is 20 mm, the distance S between the upper magnetic pole piece 1 and the lower magnetic pole piece 2 is 11 mm, the taper angle θ2 of the lower magnetic pole piece 2 is 45', and the sample position Zo is = of the lower magnetic pole piece.
It is placed 6mm above the top surface.

By the way, in an electron microscope with an accelerating voltage of 80KV to 120KV, in order to obtain a fishbone resolution of 3.4 people (to confirm the graphite crystal lattice image) and relatively strong contrast with on-axis irradiation, the spherical aberration coefficient Cs is usually used. and the axial aberration coefficient Cc are set under the following conditions.

Cs Cc 3.5 mm On the other hand, in order to obtain high contrast, an objective aperture 8 is inserted between the lower top surface of the objective lens pole piece and the sample 7. The insertion position of the objective diaphragm varies depending on the shape of the sample holder, but for example, if the sample holder is 3 mm thick and the sample is tilted by ±40°, The optimal value for the objective aperture insertion position, that is, the distance Z between the sample and the aperture, is 4.5 mm.
(4.51 points below the sample position). Therefore, if the objective lens forms the back focal point f at this position, even when using an aperture with a hole diameter of several micrometers, the sample image projected onto the fluorescent screen will not be cut off at all, and the ideal high Obtained by contrast.

However, if the distance between the sample 7 and the back focal point f, that is, the back focal length Zc is as long as 4.5 mm, the spherical aberration coefficient Cs and the axial aberration coefficient Cc increase, and the resolution deteriorates.

However, in practice, a diaphragm with a hole diameter of several tens of micrometers is often used, so if a diaphragm with a hole diameter of about 20 micrometers is used, for example, a 100 mm aperture on a fluorescent screen with a diameter of about 150 mm at a magnification of several thousand to tens of thousands of times is used. Since it is sufficient to project a sample image of 150 mm to 150 mm, it is possible to increase the resolution by reducing the distance Z between the sample and the objective aperture to about 2 mm. However, it is impossible to shorten Z in this manner due to mechanical constraints such as the sample inclination.

Therefore, the present inventor placed a 20 μm aperture at 4.5 mm from the sample position.
With the setting still set at 5 mm downwards, 100 mm to 1.
On the condition that a 50 mm sample image is projected,
FIG. 2 was obtained by experimentally varying the hole diameter b2 of the lower pole piece 2. As a result, as can be seen from Figure 2, the back focal length Z
c changed, and when the hole diameter b2 was around 8.4 mm, Zc showed the minimum value (Zc 2 mm), and the spherical aberration coefficient Cs became the minimum value (Cs 2.8 mm).

Therefore, based on these conditions, Cs≦3.5mm.

If the hole diameter b2 of the lower magnetic pole piece that satisfies Cc≦3.5 mm is determined from FIG. 2, it becomes 5.7 mm≦b2≦11 mm. When this is normalized with S=11mm, b2 is 0
．． It ranges from 5S to S.

If the hole diameter b2 of the lower magnetic pole piece is made smaller than this value, the distance Ze between the sample 7 and the back focal point f will suddenly increase, and although the contrast will improve, the spherical aberration coefficient Cs will increase rapidly, which will not meet the requirements for high resolution. is damaged.

Therefore, a practical aperture of about 20 μm should be set at 4 pm from the sample position.
．． The sample image is projected on the fluorescent screen with a practical field of view and high contrast at a magnification of approximately 5 mm below, and the conditions for maintaining high resolution are as follows: It can be seen that the distance between the magnetic pole piece 1 and the lower magnetic pole piece 2 may be set within a range from approximately 1/2 to S. At this time, the back focal point is formed at f' about 2 mm below the sample position.

[Effects of the Invention] As is clear from the above explanation, the present invention has the advantage that θ2 <
60@, 0.8S≦bl≦1.2S, Dl≧3 b 1
In the objective lens that satisfies r D 2≦3b2, by setting the hole diameter b2 of the lower pole piece between 0.5S and S, the sample is projected onto the fluorescent screen even if the objective aperture is inserted. The image is no longer subject to field cuts, and high contrast can be obtained while maintaining high resolution characteristics (grid resolution of 3.4 people with on-axis illumination).

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 shows the spherical aberration coefficient Cs when the hole diameter of the lower pole piece is varied.
FIG. 3 is a diagram showing the relationship between the axial aberration coefficient Ce and the distance Zc between the sample and the objective aperture. l: Upper magnetic pole piece 2: Lower magnetic pole piece 3.4 Ni spacer - 5 = Yoke 6: Excitation coil 7: Sample 8: Objective aperture

Claims (1)

[Claims] The diameter of the bottom surface of the upper magnetic pole piece and the diameter of the top surface of the lower magnetic pole piece are each D_
1, D_2, the hole diameters of the upper and lower magnetic pole pieces are b_1, b_2, respectively.
The spacing between the upper and lower magnetic pole pieces is S, and the taper angle of the lower magnetic pole piece is θ_2.
and θ_2<60°, 0.8S≦b_1≦1.2S,
When D_1≧3b_1, D_2≦3b_2, b_
An objective lens for a transmission electron microscope, characterized in that 2 is set to a value greater than 0.5S and smaller than S.