JPS61294746A - Scanning type electron microscope - Google Patents

Abstract

PURPOSE:To improve the resolution by employing a field radiation electron gun while arranging a sample at the inside of objective lens and arranging a secondary electron detector closely to the objective lens at the electron gun side. CONSTITUTION:FE electron gun where spreading of the energy of electron beam 8 is limited is employed while the sample 5 is arranged in the objective lens 4 in order to lower the astigmatism. While the detector 6 is arranged closely to the objective lens 4 in order to improve the detection efficiency of secondary electrons 9. For that purpose, the lens coil 41 of the objective lens 4 is arranged at the opposite side from the sample 5 against the detector 6. Consequently, a lens where spreading of the electron beam energy is limited while the astigmatism is low can be realized resulting in very fine focusing of electron beam and highly efficient detection of secondary electrons.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a scanning electron microscope (SEM), and particularly to an electron optical system suitable for ultra-high resolution secondary electron image w4 observation.

[Background of the invention]

Conventional secondary electron image resolution is highest at 20 people, and its electron optical system is described in the literature (Basics and Applications of Scanning Electron Microscopes; Kyoritsu Shuppan, December 1980, page 14, Figure 2.7)
As described in b)), the primary electron beam 8 shown in FIG. Condenser lens 3. Approximately 2 depending on objective lens 4
The sample 5 is irradiated with the electron beam diameter narrowed to zero. Sample 5
The secondary electrons 9 excited are detected by the detector 6.

The electron beam 8 is scanned two-dimensionally over the sample 9 by the deflector 7
“It will be done.

On the other hand, in recent years, as shown in FIGS. 3 and 12 of the above-mentioned document, the sample 5 was inserted into the objective lens 4 to achieve a short focus and low aberration, and the diameter of the electron beam was narrowed down to 15.

In the case of FIG. 1, since the objective lens 4 has a long focal point, the aberration is large and the electron beam 8 cannot be narrowed down. Furthermore, in the case of FIG. 2, because of the thermionic electron gun, the energy of the electron beam 8 spreads widely and chromatic aberration increases, which also has the disadvantage that the electron beam 8 cannot be narrowed down.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a scanning electron microscope equipped with an electron optical system that can narrow down the electron beam diameter to a smaller diameter than before and detect secondary electrons with high efficiency.

[Summary of the invention]

In order to achieve the above object, in the present invention, a field emission type electron gun is used as the electron gun, the sample is placed inside the objective lens, and the secondary electron detector is placed close to the objective lens on the electron gun side. The feature is that a scanning electron microscope is constructed using the following methods. That is, the present invention was made based on the following idea.

In other words, in order to focus the electron beam as narrowly as possible, it is sufficient to use an electron gun with a small energy spread and a lens system with low aberration.In addition, in order to increase the detection efficiency of secondary electrons, it is necessary to The configuration may be such that no shielding object is placed in the area.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. The electron gun 1 is an FE electron gun in which the energy spread of the electron beam 8 is small, and the sample 5 is placed inside the objective lens 4 in order to reduce aberrations.

Furthermore, in order to improve the efficiency of detecting the secondary electrons 9, the detector 6 is placed close to the objective lens 4. For this purpose, the lens coil 41 of the objective lens 4 is arranged on the opposite side of the detector 6 with respect to the sample 5.

The configuration other than the above is the same as the conventional one, and the operating principle is also the same as the conventional one.

Next, the practical shape of the magnetic pole of the objective lens 4 will be described from the viewpoint of aberrations and high detection efficiency of the secondary electrons 9. for that.

Figure 4 shows an example of an enlarged view of the magnetic pole portion of the objective lens 4, and Figure 5 shows an example of the lens magnetic field and the radius of rotation of secondary electrons.
In addition, the lens characteristics according to the magnetic pole shape are shown in the 6th to 8th figures.
As shown in the figure.

Generally, the sample stage 52 that holds the sample 51 needs to have a width of 51 or more, and since it is desired to enable an inclination angle of at least 30° in the am of the secondary electron image, the gap S between the magnetic poles 42 and 43 is required.
3m or more is required.

On the other hand, the secondary electrons excited in the sample 51 are at the magnetic poles 42°43
In order to make it difficult to collide with
It is better that bt>bt for the hole diameter b2 of the first and second magnetic poles 43. Under these conditions, a computer simulation was performed to obtain the excitation used for the spherical aberration coefficient Ca9 and the chromatic aberration coefficient C.
etc., and are shown in Figures 6-8. Here, engineering is the lens coil current.

N is the number of turns of the lens coil, and E is the acceleration voltage.

In FIGS. 6 to 8, the sample 51 is placed at the center of the lens, and b and b2 are varied under the condition of bx>bz.

This is the average value when normalized by (b1+b,)/2. From these characteristic diagrams, if you consider practicality and use it,
It becomes possible to manufacture the objective lens 4 such that C'<3 mm and C'<3 mm. If such an objective lens 4 is used, the electron beam 8 can be narrowed down to several angstroms or less with an accelerating voltage Effi of 20 KV, and a secondary electron image with extremely high resolution can be obtained.

Here, a typical objective lens (S==8 countries, b1==8
mm, b2 = 2no), on-axis magnetic field distribution 12 for acceleration voltage 20Kv and r = □ (where r is the radius of rotation of the secondary electron 9 m is the mass of the electron, e is the charge of the electron, ■ is the secondary electron energy, B indicates the lens magnetic field) Secondary electrons according to the formula 9
The radius of rotation (r) 13 of is shown in FIG. If the magnetic pole 42 is located off-axis with respect to the rotation radius 13, the secondary electrons 9 will move to the magnetic pole 42.
The secondary electrons 9 colliding with the detector 6 do not spread out, but are drawn toward the detector 6. Therefore, in practice, there is no particular problem as long as the opening angle θ of the magnetic pole 42 is 30'' or more, and such a magnetic pole 42 allows highly efficient secondary electron detection.

5 to 8 are shown with the sample 51 placed at the center of the objective lens 4, but computer simulations show that if the sample 51 is placed in the objective lens 4, it is a lens with low aberrations and high detection efficiency under the above conditions. I understand that. Therefore, the sample 5 only needs to be placed inside the objective lens 4.

〔Effect of the invention〕

According to the present invention, the spread of electron beam energy is small;
This also makes it possible to create a lens with low aberrations.

The electron beam can be focused extremely thinly.1. This has the effect of improving the resolution of scanning electron microscopes.

In addition, the detection efficiency of secondary electrons can be increased, so
S/N improvement 2 This has the effect of improving image quality.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the basic configuration of the electron optical system of a conventional SEM, FIG. 3 is a diagram showing the basic configuration of the electron optical system of the SEM of the present invention, and FIG. Figure 5 is a diagram showing the lens magnetic field and radius of rotation of secondary electrons relative to the lens magnetic pole, and Figures 6 to 8 are diagrams showing the lens characteristics of the present invention, including spherical dichromatic aberration, respectively. FIG. 3 is a diagram showing characteristics of coefficients and lens excitation. 1... Field emission type electron gun, 2... Accelerating lens, 3...
...Condenser lens, 4...Objective lens, 5...
Sample, 6...Detector, 7...Deflector, 8...-Blow mold beam, 9...Secondary electron, 11...Thermionic gun, 12
... Lens magnetic field, 13 ... Rotation radius of secondary electron, 4
1... Lens coil, 42... First magnetic pole, 43...
・Second magnetic pole, 51...sample, irises 1 Figure
Stone 2 Figure Summer 3 Figure Release 4 Figure fJ 7 Figure 2 and Jukuki no Hyaku 3 Figure

Claims (1)

[Claims] 1. A scanning type in which an electron beam taken out from an electron gun and narrowed down is scanned two-dimensionally over a sample, and secondary electrons excited within the sample are detected and displayed on a cathode ray tube. In the electron microscope, a field emission electron gun is used as the electron gun, the sample is placed inside an objective lens, and a detector for detecting secondary electrons coming out of the sample is placed close to the objective lens. A scanning electron microscope characterized by being placed on the electron gun side. 2. The objective lens is arranged such that its lens coil is on the opposite side of the sample from the detector, and the magnetic pole shape of the lens magnetic path has a gap S of 3 mm≦S≦12 mm.
, with respect to the first magnetic pole diameter b_1 on the electron gun side and the magnetic pole diameter b_2 on the lens coil side, b_1>b_2, 2mm≦(
2. The scanning electron microscope according to claim 1, wherein b_1+b_2)/2≦16 mm, and the half-opening angle θ_1 of the first magnetic pole is θ_1≧30°.