JPH0232738B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for optimally maintaining a sample position and an objective lens aperture position in a transmission imaging electron microscope.

In conventional transmission imaging electron microscopes, image observation is performed with the distance between the objective lens and the sample fixed. In other words, the conventional sample movement mechanism is
Even if you move the sample horizontally in a plane perpendicular to the objective lens optical axis (Z-axis) or tilt it to the optical axis to change the field of view of the microscope image, the position where the sample intersects with the optical axis will change. It was not possible to do so and remained constant. However, when trying to obtain a higher resolution microscope image, the spherical aberration coefficient of the objective lens
In order to maintain Cs at an optimal value depending on observation conditions, it is required to fix the excitation intensity of the objective lens to a certain value. (For example, patent application 1984-141028)
In this case, it is not possible to change the excitation of the objective lens in order to focus the microscope image, so it is necessary to move the position of the sample along the optical axis Z to the object plane position of the objective lens. However, in this method of focusing by moving the sample in the optical axis direction, the objective lens diaphragm cannot fully perform its original function, and the contrast of the image tends to decrease. Confirmed by the inventor.

The purpose of the present invention is to solve these problems and maintain the appropriate contrast of the microscope image even when the sample is moved in the direction of the optical axis of the objective lens. a sample moving mechanism that moves the sample position in the optical axis direction; an objective lens aperture moving mechanism that moves the aperture position of the objective lens in the optical axis direction; and the objective aperture moving mechanism is moved by the operation of the sample moving mechanism. The present invention is characterized in that it includes means for automatic control based on a signal representing a changing sample position.

FIG. 1 is a schematic diagram showing the configuration of an apparatus according to an embodiment of the present invention. In the figure, reference numeral 1 indicates the peripheral portion of the objective lens 2 in the vacuum lens barrel of a transmission imaging electron microscope. The objective lens 2 is composed of a lens coil 3 wound around the lens optical axis Z, a yoke 4 surrounding it, and two magnetic pole pieces 5 and 6, and an objective lens power source 7.
When the lens coil 3 is excited by this, a strong lens magnetic field is formed between the two magnetic pole pieces 5 and 6. A sample holder 8 and an aperture holder 9 are inserted into the lens magnetic field from a direction perpendicular to the optical axis through the lens barrel 1 and yoke 4, and their positions, including movement in the Z-axis direction, are controlled by This is performed by a sample moving mechanism 10 and an aperture moving mechanism 11. The Z-axis position signal of the sample detected from the sample moving mechanism 10 is constantly monitored by the central controller 12, and when the position changes, the optimal Z-axis position of the objective lens aperture is determined for the new sample position. is calculated and generates a control signal to the aperture position control circuit 13 to move the aperture moving mechanism 11.
is configured to control.

FIG. 2 shows the positional relationship between the sample 14 held at the tip of the sample holder 7, the objective lens diaphragm 15 held at the tip of the aperture holder 9, and the objective lens magnetic field main surface 16. . In this figure, aperture 1
5 is in an ideal position, the position of the diffraction image plane formed by the electron beam EB incident approximately parallel to the sample coincides with the position of the aperture 15. In this state, the transmitted electron beam that has been scattered or has lost energy in the sample 14 is blocked by the diaphragm 15, as shown by E1 in the figure, and is therefore stopped by the imaging lens system placed after the objective lens. Sufficient contrast can be obtained in the microscopic image formed by this method.

In Figure 3, the lens strength of the objective lens is weakened and the position of the sample 14 is moved from S1 to S2 in Figure 2, while the distance A1 of the aperture 14 from the objective lens main surface 16 remains unchanged. Indicates the status of In this state, the diffraction image plane position D formed by the electron beam EB does not match the position of the aperture 15 and shifts downward. As a result, a part of the electron beam that has passed through the sample 14 without energy loss or scattering is blocked by the aperture 15 from passing through the sample 14, so the brightness of the microscope image decreases and a clear microscope image is obtained. become unable to form. Therefore, in this case, it is necessary to shift the diaphragm 15 downward to match the position of the diffraction image plane at a distance of A2 from the principal surface 16 of the objective lens, ie, the optimum position, as shown in FIG. In conventional devices, the position of the objective lens diaphragm 15 in the optical axis direction is fixed, but in the embodiment shown in FIG. 1, the position is varied and set to the optimum state as shown in FIG. 4. be able to.

In order to set the optimum condition as shown in Figure 4, first set the spherical aberration coefficient of the objective lens.
The output of the lens power supply 7 is adjusted to the objective lens excitation intensity to obtain Cs. Next, the microscope image formed on the fluorescent screen is observed, and if it is not in focus, the sample moving mechanism 10 is manually operated to adjust the focus to be in focus. When an electric signal related to the sample position is applied to the central controller 12 through such an adjustment operation, the diffraction image plane position A2 in FIG. 4 is calculated together with the focal length signal of the objective lens, and a control signal representing the result is calculated. is applied to the aperture position control circuit 13, and the position of the aperture 15 on the optical axis is moved via the aperture moving mechanism 11 to the optimum position. The signal representing the focal length of the objective lens is obtained from a signal corresponding to the output of the lens power source 7 and an accelerating voltage signal of the sample irradiating electron beam EB applied from the terminal 17 to the central controller 12.

By the way, the sample-transmitting electron beam used to form a microscopic image may not only be an electron beam that is not scattered in the sample, but also a specific diffracted electron beam that is scattered in the sample depending on the purpose of observation. In some cases, an objective lens aperture 15a having a large hole diameter P is used as shown in FIG. In such an observation mode, the excitation of the objective lens is changed (decreased), and the sample position is moved from S1 to S2 for focus adjustment accordingly, and at the same time, the position of the objective lens aperture 15a is changed as in the case of Fig. 4. FIG. 6 shows the electron beam path when moving from A1 to A2. As is clear from FIG. 6, even if the objective lens diaphragm 15a is placed at the correct position on the optical axis Z, if the aperture P of the objective lens diaphragm 15a is constant, the light passing through the diaphragm in FIG. Since a part of the diffracted electron beam that was previously being used no longer passes through, the contents of the microscope image obtained in the state shown in FIG. 5 are different from the contents of the microscope image obtained in the state shown in FIG. In order to eliminate this difference, it is necessary to use an objective lens aperture having a larger aperture diameter Q, as shown in FIG.
In an ordinary electron microscope, several types of objective lens apertures with different hole diameters are prepared.
Since a means for exchanging and using them is built into the aperture moving mechanism, this exchanging mechanism can be operated to bring the aperture closer to the optimal state as shown in FIG. In addition, the apparatus of the embodiment shown in FIG. 1 not only automatically controls the diaphragm position, but also calculates the diaphragm aperture diameter to obtain the above-mentioned optimum condition, and uses an objective lens having an aperture diameter as close as possible to the result of the calculation. It is also easy to automatically replace the aperture.

As detailed above, in the device of the present invention,
A sample moving mechanism that moves the sample position in the optical axis direction of the objective lens, an objective lens aperture moving mechanism that moves the aperture position of the objective lens in the optical axis direction, and the objective aperture moving mechanism is used to operate the sample moving mechanism. The system is equipped with means for automatic control based on signals representing the changing sample position, so even if the sample position changes, the brightness and contrast of the microscope image is suppressed and the microscope image is always clear. Observations can be made.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining an embodiment of the present invention, and FIGS. 2 to 7 are schematic diagrams for explaining the principle of the present invention. 1: Lens barrel, 2: Objective lens, 3: Lens coil, 4: Yoke, 5, 6: Magnetic pole pieces, 7: Lens power supply, 8: Sample holder, 9: Aperture holder, 10: Sample moving mechanism, 11 : Aperture moving mechanism, 12: Central controller, 13: Aperture position control circuit, 14: Sample,
15, 15a, 15b,: Objective lens aperture, 1
6: Objective lens main surface, 17: Terminal.

Claims (1)

[Claims] 1. A sample moving mechanism that moves the sample position in the optical axis direction of the objective lens, an objective lens aperture moving mechanism that moves the aperture position of the objective lens in the optical axis direction, and the objective aperture moving mechanism is operated by the sample moving mechanism. 1. A transmission imaging electron microscope characterized by comprising means for automatic control based on a signal representing a sample position that changes depending on the conditions.