JPS59181447A - Shaft slip compensator for electromagnetic lens - Google Patents

Abstract

PURPOSE:To make the position of a lens magnetic field itself shiftable and adadjustable in an easy manner, by installing a device to feed a magnetic field generating coil with an exciting current so as to cause light in the direction of a compensating magnetic field formed by the magnetic field generating coil to be paralleled with an optical axis and reversely opposed with each other. CONSTITUTION:A sample 5 being held by a sample holder 4 is set up on a main surface 3 of an objective lens magnetic field, while eight ringlike compensating magnetic field generating coils 6 are installed in a symmetrical position with an optical axis 2 on top of the main surface 3. Coils being opposed with each other among these coils 6 are connected in series, and since a sense of winding in these coils is reversed, components in parallel with a Z-axis on the optical axis Z become opposed with each other in direction. Therefore, an exciting current for these coils being set up like this is adjusted to what it should be, whereby a peak of the lens magnetic field comes possible to be brought onto the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a lens magnetic field correction device that can be incorporated into an electromagnetic lens used in a transmission electron microscope or the like to obtain a large effect.

In an electron beam device such as an electron microscope that uses a large number of electromagnetic lenses, it is necessary to align the optical axes of each electromagnetic lens. Ideally, this alignment would be done by mechanically moving and adjusting the arrangement of each lens, but the mechanism would be complicated and large, so electron beams are used as an electrical axis alignment method. Deflection means are often used.

However, when alignment is performed using such a deflection means, in the case of an electron microscope, a phenomenon occurs in which the axes called the voltage axis and current axis of the objective lens do not coincide with the optical axis of the lens system. arise. The voltage axis refers to the center at which the microscope image formed on the fluorescent screen expands and contracts as it rotates when the accelerating voltage of the electron beam is periodically varied, and the current axis refers to the center at which the excitation current of the objective lens is periodically varied. This is the center at which the microscope image on the fluorescent screen expands and contracts while rotating when the target changes. These voltage axes.

Since the amount of variation in a microscope image that expands and contracts around the current axis increases as it moves away from the axis, if these axes do not coincide with the axis of the lens system, a high quality microscope image cannot be obtained. This is because the axis of the lens optical system coincides with the center of the fluorescent screen, so if this center is not aligned with the voltage and current axes, which have the least variation in the microscope image, the electrons displayed on the fluorescent screen will This is because the microscopic image is greatly affected by current fluctuations and voltage fluctuations. The stability of voltage and current LJ in an actual electron microscope is extremely high at about 10-6, but in image observation where resolution is an issue with high magnification images, this axis deviation is The phenomenon is that the image is blurred!
This is not desirable as it may cause you to get angry. However, in principle, it is impossible to precisely align the voltage axis and the current axis with +tl+ of the lens system when performing an alignment operation using a deflection means.

In view of these points, the present invention provides alignment using a mechanical movement mechanism of a λ'' object lens or an electron beam deflection device N.
The purpose of this device is to move and adjust the position of the lens magnetic field itself by providing a magnetic field correction means, which includes a plurality of magnetic field generating coils arranged at positions [d] on or near the main surface of the lens and symmetrically with respect to the optical axis of the lens. and means for supplying excitation current to the magnetic field generating coils so that the directions of the correction magnetic fields formed by the magnetic field generating coils facing each other across the optical axis are parallel to the optical axis and opposite to each other. Features and covers.

Generally formed by an electromagnetic lens! Of the fried
It is the magnetic field component B7 in the optical axis Z direction that contributes to the lens magnetic field, and the intensity distribution of this 3z is as follows: Z is the coordinate of the 7th axis, r is the distance from the 7th axis, and θ is the azimuth around the 7th axis. Using a cylindrical coordinate system (r, θ, 2) with angles, it can be expressed as follows.

Here, φ1. φ2. φ3 is a function determined by the shape of the lens magnetic pole piece, δ1. δ2. δ3 is a constant determined by the shape of the lens pole piece and the uniformity of the material. The first line on the right side of the above equation represents an axis-symmetrical magnetic field component, the second line corresponds to an axis-offset component, and the third and subsequent lines correspond to second-order and third-order astigmatic components. There is no problem with regard to second-order astigmatism, which is currently being corrected by a stigma correction device, and third-order and higher-order astigmatism is hardly considered a problem because its influence is small.

However, as described above, the axis misalignment cannot be corrected even by using the deflection means, and is often dependent on the processing accuracy of the lens pole piece and the uniformity of the material. the above(
1) Extract the axially symmetrical component and the axially misaligned component in equation
By omitting the higher-order terms and rewriting Bz with θ constant, we get the following.

Bz(T')=, 4-Bl'+CY" From this equation, a quadratic curve with a minimum value or position 7 at the optical axis - in the objective 1 nons is shown, and the value of Bz determined by the shape of the magnetic pole piece. It can be seen that the amount of axis misalignment is determined by the value of φ1 (z) determined by the processing viscosity of the magnetic pole piece, etc. This distribution state has been confirmed by experiments, and an example of this is shown in the first example. In an experimental example using an electron microscope with an accelerating voltage of 200 KV, the value of [3z'', which is the coefficient of the quadratic term in the equation representing the quadratic curve described above, is 800 Gauss/mm.
It was about 2.

FIG. 2 shows a schematic diagram of an apparatus according to an embodiment of the present invention, in which numerals 1 and 2 indicate the upper and lower pole pieces of the objective lens, respectively. A sample 5 held by a sample holder 4 is installed on the main surface 3 of the objective lens magnetic field, and eight ring-shaped correction magnetic field generating coils 6 are installed at positions symmetrical to the optical axis Z on the main surface. has been done. FIG. 3 is a view of the coil 6 viewed from the optical axis direction. The coils that face each other among these coils are connected in series, and the winding directions of the coils are reversed, so that the components parallel to the Z-axis on the optical axis 7j are in opposite directions. Such a correction magnetic field is the main fe formed in conventional deflection devices and astigmatism correction devices.
This is a big difference compared to the case where the field component was perpendicular to the optical axis. When any pair of coils 6 facing each other is excited, the direction θ connecting these coils (
The magnetic field distribution in the r direction is expressed by the following equation.

However, 11 o is the vacuum permeability, I is the current value to the coil, N LJI is the number of turns per coil, D is a constant, and a and l) respectively fall on the cross section of the coil shown in Figure 4. Represents the length of a specific point. When the above equation is expanded in the vicinity of the optical axis Z (“-〇), the constant term and the squared term disappear, and if higher-order terms are omitted, it is transformed as follows, and the state of the magnetic field distribution is as shown in Figure 4. That's how it is.

Therefore, by adjusting the excitation current I of the coil arranged in this way, the slope of the curve shown in Fig. 1 can be changed arbitrarily, so the curve shown in Fig. 4 can be changed to In addition to the curve shown in the figure, it is now possible to bring the peak of the lens magnetic field onto the optical axis. This cancels the ■r1 of Br near the optical axis, moves the point where BZ(r) is maximum or minimum onto the optical axis, and the light [111
This means that it is possible to create a magnetic field that is axially symmetrical with respect to the mouth.

If the axis of the objective lens magnetic field is aligned using the h method,
There is no longer a phenomenon in which the current axis cannot be aligned with the optical axis even if the voltage axis is aligned with the optical axis as in the conventional case.

It should be noted that the present invention is not limited to the above-described embodiments, and it is also possible to use 4, 12, or more magnetic field correction coils, and the installation position of the coils may be slightly different from the objective lens. Even if it deviates from the main aspect, it is not a big problem.

As described above, according to the present invention, since it is possible in principle to align the voltage axis and the current axis with the optical axis, it can be used as a device for adjusting lenses in transmission electron microscopes or other electron beam devices. It's a big effect, you get 1q.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the mechanical optical axis of the electromagnetic lens and the actual magnetic field distribution curve, Figs. 2 and 3 are schematic diagrams showing the configuration of the device of the present invention, and Fig. 4 is a diagram showing the correction magnetic field by the device of the present invention. FIG.

Claims (1)

[Claims] A plurality of magnetic field generating coils arranged at symmetrical positions with respect to the optical axis on or near the main surface of the lens, and the magnetic field generating coils facing each other with the optical axis in between, form a correction magnetic field in the direction of the optical tll+. 1. An electromagnetic lens axis misalignment correcting device comprising: means for supplying excitation current to the magnetic field generating coil in parallel to and opposite to each other.