JPH0515029B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a crystal sample observation method using an electron microscope, and more specifically to an observation method that suppresses changes in a microscope image due to changes in accelerating voltage of an electron beam irradiating the sample. Regarding.

[Prior Art] There are various methods of observing a crystal sample using an electron microscope, and one of them is a method of observing an electron microscope image using an electron beam diffracted in a specific direction.
FIG. 1 shows the vicinity of the optical axis of the objective lens in such observation. In FIG. 1, a crystal sample 1 inserted between objective lens pole pieces (not shown) is irradiated by an electron beam 3 incident parallel to the optical axis 2. The transmitted electron beam 4 diffracted by the crystal sample 1 forms a diffraction pattern on the focal plane 6 by the objective lens magnetic field 5. An objective lens diaphragm plate 7 is inserted into the back focal plane 6 so as to be movable in a direction perpendicular to the optical axis 2, and the relationship between the objective lens diaphragm plate 7 and the diffraction pattern when viewed from the optical axis direction is shown in Figure 2. Expressed in As shown by dashed lines A, B, and C in FIG. After adjustment is made so that only the diffraction spot passes through, the beam is introduced into an imaging lens system consisting of an intermediate lens and the like, and a microscopic image is formed on a fluorescent screen.

[Problems to be Solved by the Invention] However, if an attempt is made to switch the accelerating voltage of the electron beam irradiating the sample, which is one of the observation conditions, during this observation, a problem occurs in image observation. In other words, recent electron microscopes perform relativistic correction when the accelerating voltage of the electron beam is changed, and automatically adjust the excitation of each electron lens so that the focal length of the electron beam remains constant. Even with such automatic adjustment, when observing a diffraction pattern, the diffraction angle θ in the crystal sample changes due to the change in the wavelength of the electron beam, so the position adjustment of the objective lens aperture plate 7 shown in FIG. As a result, the position of a particular diffraction spot that was being captured moves, making it impossible to maintain constant observation conditions and requiring readjustment.

The present invention solves the above-mentioned problems and enables the observation of crystal samples using an electron microscope, which allows electron beams related to specific diffraction spots to be continuously extracted from the aperture hole without adjusting the objective aperture plate even when the accelerating voltage is changed. The purpose is to provide an observation method.

[Method for solving the problem] Therefore, the present invention provides a method for changing the wavelength of the electron beam irradiating the sample to λ.
When the focal length of the objective lens with respect to the sample irradiation electron beam is f, the excitation of the objective lens is controlled so that f·λ is constant regardless of changes in the accelerating voltage of the sample irradiation electron beam, and the excitation of the objective lens is This method is characterized by a crystal sample observation method using an electron microscope in which the sample is moved to the front focal plane of an objective lens that has been moved by changing the .

[Operation of the Invention] In FIG. 1, numeral 8 represents the image plane position by the objective lens, and the excitation adjustment of the objective lens is normally performed so that the image plane position does not change. Now, the focal length of the objective lens for the electron beam is f, and the distance between the sample 1 and the main surface of the objective lens is a (this distance is slightly longer than the focal length f of the objective lens, but is usually considered to be equal to f. ), and the distance between the principal surface of the objective lens and the image plane is b, the following equation (1) holds true.

1/a+1/b=1/f...(1) Figure 3 shows the relationship between the distance R of one diffraction spot from the optical axis 2 at the back focal plane 6 where the diffraction pattern is formed and its diffraction angle 2θ. This is a schematic diagram showing the relationship shown in this diagram, and the following equation (2) holds true.

R=2f·tan2θ (2) Furthermore, if the interplanar spacing of the sample crystal is d, the following equation (3) holds true from Bragg's equation.

2d·sinθ=λ (3) Since θ≪1, sinθ=θ and tan2θ=2θ, and the following equation (4) is derived from equations (2) and (3).

R=2f・λ/d...(4) In order to prevent the positional relationship between the diffraction pattern and the objective aperture plate from changing due to changes in the accelerating voltage of the electron beam, the value of R in equation (4) must be constant. It is necessary that However, since the lattice spacing d is always constant, it can be seen that in order to keep R constant, it is sufficient to keep f·λ constant.

[Example] Now, if the wavelength of the electron beam when the accelerating voltage is changed from a certain value is λ', then the value f' of the focal length f corresponding to this wavelength change is determined. Therefore, it is necessary to adjust the excitation intensity of the objective lens according to this f'. Furthermore, it is necessary to move the insertion position of the sample to the front focal plane of a new objective lens so that the value a' of a when f in equation (1) is replaced with f' is satisfied. Note that since the back focal plane moves as the focal length changes, it is conceivable to move the objective lens diaphragm plate 7 in the optical axis direction in accordance with the movement of the back focal plane. Since the change in the diffraction spot diameter due to the aperture is small compared to the diameter of the aperture hole, its influence is so small that it can be ignored. Therefore, the aperture plate 7 does not necessarily have to be moved in practice.

The above explanation is based on the principle, but in order to implement it in an actual device, f/
It is preferable to experimentally determine and record the sample position and objective lens excitation intensity that satisfy the condition that λ is constant. When switching the accelerating voltage, the vertical movement mechanism and objective lens excitation adjustment means built into the sample movement mechanism are operated to adjust the sample position and objective lens intensity recorded in advance to the recorded values. The method of the present invention can be easily carried out by adjusting the following.

[Effect] As described above, according to the present invention, even if the accelerating voltage is changed, the movement of the diffraction spot to be focused on in the direction perpendicular to the optical axis can be suppressed, and therefore the position adjustment of the objective aperture can be omitted. , it is possible to improve workability when observing a crystalline sample while switching the accelerating voltage.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram to explain the crystal sample and the electron beam path near the objective lens, Figure 2 is a diagram to explain the relationship between the electron beam diffraction pattern and the objective aperture plate, and Figure 3 is a diagram to explain electron beam diffraction. This is a schematic diagram for 1: sample, 2: optical axis, 3: sample irradiation electron beam,
4: Objective lens, 5: Sample transmitted electron beam, 6: Back focal plane, 7: Objective lens aperture plate, 8: Objective lens image plane.

Claims (1)

[Claims] 1. When the wavelength of the electron beam irradiating the sample is λ and the focal length of the objective lens with respect to the electron beam irradiating the sample is f, the objective lens is 1. A method for observing a crystal sample using an electron microscope, which comprises controlling the excitation of f and moving the sample to the front focal plane of an objective lens that has been moved by changing f.