JPH0654646B2 - Objective lens in transmission electron microscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an objective lens used in a transmission electron microscope, and more particularly to a shape of an objective lens pole piece capable of simultaneously obtaining high resolution and high contrast.

[Prior Art] Conventionally, as an objective lens of a transmission electron microscope capable of observing a low-magnification image of a biological slice or the like with good contrast, there is known one having a structure disclosed in JP-A-61-4143. 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 D ₁ and D ₂ , respectively, and the hole diameters of the upper and lower magnetic pole pieces are b ₁ and b ₂ , respectively. The interval is S,
When the taper angle of the lower magnetic pole piece is θ ₂ , θ ₂ <60 ° in an electron lens, 0.8S ≦ b ₁ ≦ 1.2
S, D ₁ ≧ 3b ₁ , 0.3S <b ₂ ≦ 0.5S and D ₂
It has been proposed to form the upper and lower pole pieces so as to satisfy ≦ 3b ₂ .

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

An object of the present invention is to provide an objective lens capable of simultaneously obtaining high resolution and high contrast in consideration of such points.

[Means for Solving the Problems] In the present invention, 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 D ₁ and D ₂ , respectively, and the hole diameters of the upper and lower magnetic pole pieces are b ₁ , respectively.
b ₂ , the distance between the upper and lower magnetic pole pieces is S, and the taper angle of the lower magnetic pole piece is θ ₂ , θ ₂ <60 °, 0.8S ≦ b ₁ ≦ 1.2S,
When D ₁ ≧ 3b ₁ and D ₂ ≦ 3, b ₂ is set to a value larger than 0.5S and smaller than S.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. First
FIG. 1 is a configuration diagram of an objective lens for explaining an embodiment of the present invention. In FIG. 1, 1 is the upper magnetic pole piece, b ₁ is the hole diameter of the upper magnetic pole piece 1, and D ₁ is the diameter of the bottom surface of the upper magnetic pole piece 1.
2 is a lower magnetic pole piece, b ₂ is a hole diameter of the lower magnetic pole piece 2, and D ₁ is a 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, θ ₂ is the taper angle of the lower magnetic pole piece 2,
3 and 4 are spacers for supporting the pole pieces, and 5
Is a yoke, 6 is an exciting coil, 7 is a sample, and 8 is an objective diaphragm.

The inventor of the present invention has proposed the above-mentioned Japanese Patent Application Laid-Open No. 61-61
As a result of considering the parameters of the proposed objective lens disclosed in Japanese Patent No. 4143, b ₁ , b ₂ , D ₁ , D ₂ , S, θ ₂ , other conditions (b ₁ , D ₁ , D ₂ , S, θ
_While keeping ₂ ), it was found that b ₂ largely contributes to the spherical aberration coefficient Cs and the axial chromatic aberration coefficient Cc that uniquely determine the resolution.

Therefore, the taper angle θ ₂ of the lower magnetic pole piece ₂ is θ ₂ <60 ° and the hole diameter of the upper magnetic pole piece 1 is 0.8S ≦, which are structural conditions.
b ₁ ≦ 1.2S, the diameter of the bottom surface of the upper magnetic pole piece 1 is D ₁ ≧ 3
b ₁ , the diameter of the top surface of the lower pole piece ₂ is maintained at D ₂ ≦ 3b ₂ ,
The varying the hole diameter b ₂ below pole pieces 2 was tried experimentally.

FIG. 2 shows the hole diameter b ₂ of the lower pole piece 2 by the lens of FIG.
FIG. 9 is a diagram showing the relationship among b _{2 in} the case where V is varied, the spherical aberration coefficient Cs, the axial chromatic aberration coefficient Cc, and the distance Zc between the sample and the back focal point. At this time, the hole diameter b ₁ of the upper magnetic pole piece is 10 mm, The diameter D ₁ of the bottom surface is 36 mm, the diameter D _{2 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 θ ₂ of the lower magnetic pole piece ₂ is 45 °, and the sample position Z ₀ is It is located 6 mm above the top surface of the lower pole piece.

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

Cs Cc 3.5 mm On the other hand, in order to obtain high contrast, the objective diaphragm 8 is inserted between the lower pole top surface of the objective lens pole piece and the sample 7. The condition of the insertion position of the objective diaphragm changes depending on the shape of the sample holder. However, if the sample holder having a thickness of 3 mm is used to incline the sample by ± 40 °, the inclination of the sample may be prevented. There is no objective aperture insertion position, that is, the optimum value of the distance Z _a between the sample and the aperture is 4.5 mm.
(4.5 mm below the sample position). Therefore, if the objective lens is such that the rear focal point f is formed at this position,
Even when a diaphragm with a hole diameter of several μm is used, the sample image projected on the fluorescent plate is obtained with ideal high contrast without any field cut.

However, if the distance between the sample 7 and the rear focal point f, that is, the rear 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 about several tens of μm is often used. Therefore, for example, when a diaphragm of about 20 μm is used, it is 100 mm on a fluorescent plate with a diameter of about 150 mm at a magnification of several thousand to several tens of thousands. or because sample image of 150mm need be projected distance Z _a of the objective aperture of the sample it is possible to increase the resolution and reduced to about 2 mm. However, it is impossible to shorten Z _a in this way due to mechanical restrictions due to the sample inclination and the like.

Therefore, the present inventor set a diaphragm of 20 μm from the sample position to 4.
With the setting of 5 mm downward, 100 mm to 1 on a fluorescent plate with a diameter of 150 mm at a magnification of 10,000 times.
FIG. 2 was obtained by experimentally varying the hole diameter b ₂ of the lower magnetic pole piece 2 under the condition that a sample image of 50 mm was projected.
As a result, the rear focal length Zc As can be seen from Figure 2 changes, hole diameter _{b 2} is Zc is minimum value in the vicinity of 8.4 mm (Z
c 2 mm) and the spherical aberration coefficient Cs became the minimum value (Cs 2.8 mm).

Therefore, from these conditions, Cs ≤ 3.5 mm, Cc ≤
When the hole diameter b ₂ of the lower magnetic pole piece satisfying 3.5 mm is obtained from FIG. 2, it is 5.7 mm ≦ b ₂ ≦ 11 mm. If this is normalized with S = 11 mm, b ₂ is 0.5S.
The range is from S to S.

When the hole diameter b ₂ of the lower magnetic pole piece is set to a smaller value than this, the distance Zc between the sample 7 and the back focal point f increases sharply, and the contrast improves, but the spherical aberration coefficient Cs increases sharply, resulting in high resolution. Conditions are spoiled.

Therefore, a practical aperture of about 20 μm is placed 4.5 mm below the sample position, and the sample image is projected on the fluorescent plate with a practical field of view and high contrast at a magnification of about 10,000 times, and the high resolution is increased. The condition for maintaining is that the hole diameter b ₂ of the lower magnetic pole piece is approximately 1/2 of the distance S between the upper magnetic pole piece 1 and the lower magnetic pole piece 2.
It is understood that it is sufficient to set the range from S to S. At this time, the rear focal point is formed at f ′ which is about 2 mm below the sample position.

[Effects of the Invention] As is clear from the above description, the present invention is characterized by θ ₂ <6.
0 °, 0.8S ≦ b ₁ ≦ 1.2S, D ₁ ≧ 3b ₁ , D ₂
In the objective lens satisfying ≦ 3b ₂ , by forming the hole diameter b ₂ of the lower pole piece to be set between 0.5S and S, the sample image projected on the fluorescent plate even if the objective diaphragm is inserted. Does not suffer from field cut, and high contrast can be obtained while maintaining high resolution characteristics (3.4Å lattice resolution with on-axis irradiation).

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a spherical aberration coefficient Cs when the hole diameter of the lower pole piece is variously changed,
It is a figure for showing the relation of axial aberration coefficient Cc and interval Zc between a sample and an object diaphragm. 1: Upper magnetic pole piece, 2: Lower magnetic pole piece 3,4: Spacer, 5: Yoke 6: Excitation coil, 7: Sample 8: Objective diaphragm

Claims (1)

[Claims] 1. 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 D ₁ and D ₂ , respectively, and the hole diameters of the upper and lower magnetic pole pieces are b ₁ and b, respectively.
_2. The distance between the upper and lower pole pieces is S, and the taper angle of the lower pole piece is θ.
₂ and θ ₂ <60 °, 0.8S ≦ b ₁ ≦ 1.2S, D
_{When 1} ≧ 3b ₁ and D ₂ ≦ 3b ₂ , b ₂ is 0.5S
An objective lens in a transmission electron microscope, which is set to a value larger than S and smaller than S.