JPH03219542A - Exciting system of electromagnetic lens - Google Patents

Abstract

PURPOSE:To minimize spherical aberration coefficient at the time of high acceleration and chromatic aberration coefficient at the time of low acceleration and increase space resolution by providing an electromagnetic lens having two coils disposed therein and respective coil driving devices, and determining exciting currents by a control device. CONSTITUTION:An objective magnetic pole 1 has coils 2, 3 disposed in a gap, and a control means 4 determines exciting currents according to the voltage value from an accelerating voltage setting means 7 and instructs them to driving devices 5, 6, which supply the exciting currents to the coils 2, 3, independently. The coils form lines of magnetic force 9, 10, respectively, which are synthesized to form a determined magnetic field distribution. A table for exciting current to accelerating voltage on the basis of form of the magnetic pole 1 and constitution and arrangement of coils is preliminarily stored in the control device 4, and the exciting currents of the coils 2, 3 are determined in reference to the table. To minimize chromic aberration coefficient at the time of low accelerating voltage, the current of the coil 3 is increased to form a narrow magnetic field distribution 26, and to minimize spherical aberration coefficient at the time of high accelerating voltage, the current of the coil 2 is increased to a wide distribution 25, whereby the aberration can be minimized to improve space resolution.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to an electron microscope or an X-ray microanalyzer (EPMA).
Regarding electromagnetic lenses used in devices that irradiate samples with electron beams, such as ay Micro Analyzers,
In particular, the excitation method % formula % [Prior art] In devices such as electron microscopes and X-ray microanalyzers that irradiate a sample with an electron beam and perform enlarged image observation and various analyzes etc., the spatial resolution is determined by the accelerating voltage. It is known that when the accelerating voltage is high, it is determined approximately by the spherical aberration coefficient, and when the accelerating voltage is low, it is approximately determined by the chromatic aberration coefficient.

It is also known that there is a relationship between the spherical aberration coefficient and the chromatic aberration coefficient, such that if one is made smaller, the other becomes larger, and it is not possible to minimize both coefficients.

However, various proposals have been made to reduce the spherical aberration coefficient and chromatic aberration coefficient and thereby improve spatial resolution.
For example, Japanese Patent Publication No. Sho Go-12739 discloses that by moving the magnetic pole pieces along the optical axis, the magnetic pole spacing and, by extension, the half-width of the magnetic field are changed, and this is done in response to changes in the accelerating voltage. It has been shown that the spherical aberration coefficient is minimized during high acceleration, and the chromatic aberration coefficient is minimized during low acceleration.

[Problems to be Solved by the Invention] However, in the method disclosed in Japanese Patent Publication No. Sho GO-12739, it is extremely difficult to move the magnetic pole piece while maintaining the machining accuracy required for the magnetic pole piece. It's not realistic.

Therefore, in conventional scanning electron microscopes and the like, appropriate spatial resolution has only been obtained by finding an excitation current that minimizes the spherical aberration coefficient or chromatic aberration coefficient, or by finding a compromise between the two.

The present invention solves the above problem, and improves spatial resolution by minimizing the spherical aberration coefficient during high acceleration and minimizing the chromatic aberration coefficient during low acceleration, without moving the magnetic pole of the electromagnetic lens. The purpose of this invention is to provide an excitation method for an electromagnetic lens that can be used.

[Means for Solving the Problems] In order to achieve the above object, the electromagnetic lens excitation method of the present invention provides an electromagnetic lens in which at least two coils, a first coil and a second coil, are disposed, and A first drive device that supplies an excitation current to one coil, a second drive device that supplies an excitation current to the second coil, and a control device, the control device controlling at least one of the first coils according to an acceleration voltage.
The method is characterized in that an excitation current to be supplied to the coil and the second coil is determined.

[Operation and Effects of the Invention] In the present invention, two or more coils arranged at the magnetic poles of an electromagnetic lens are excited by independent drive devices, and the ratio of the exciting currents is changed according to the accelerating voltage. Therefore, it is possible to generate a magnetic field distribution that minimizes the spherical aberration coefficient at high acceleration and the chromatic aberration coefficient at low acceleration, and there is no need for moving parts, which would be a problem in practice.

In fact, when the present invention is applied to an objective lens, the spherical aberration coefficient is not worse than that of a conventional objective lens during high acceleration, and the chromatic aberration coefficient is about 60% of that of a conventional objective lens during low acceleration. It has been confirmed that it is possible to reduce the

[Example] Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an embodiment in which the electromagnetic lens excitation method according to the present invention is applied to an objective lens. In the figure, 1 is the magnetic pole of the objective lens, 2 is the first coil, and 3 is the magnetic pole of the objective lens. Second
4 is a control device, 5 is a first drive device, 6 is a second drive device, 7 is an acceleration voltage setting means, and O is an optical axis. Note that the cross section of the objective lens is shown. In addition, the sample, objective aperture, etc. are omitted.

In FIG. 1, an objective lens magnetic pole 1 has two gaps, and a first coil 2 and a second coil 3 are arranged between each gap. The control device 4 determines respective excitation currents to be supplied to the first coil 2 and the second coil 3 according to the acceleration voltage value taken in from the acceleration voltage setting means 7, and controls the first drive device 5 and the second drive device. 6.

The first drive device 5 supplies the instructed excitation current to the first coil 2, and the second drive device 6 supplies the instructed excitation current to the second coil 3. That is, the excitation currents of the first coil 2 and the second coil 3 are supplied independently from each other. As a result, the first coil 2 generates lines of magnetic force as shown by 9 to generate a predetermined magnetic field distribution, and the second coil 3 generates lines of magnetic force as shown by 10 to generate a predetermined magnetic field distribution. The two magnetic fields are combined to form a predetermined magnetic field distribution.

In order to determine the excitation current to be supplied to the first coil 2 and the second coil 3 according to the acceleration voltage value taken in from the acceleration voltage setting means 7, for example, by referring to a table as shown in FIG. It can be carried out. That is, in FIG. 2, the excitation current of the first coil 2 with respect to the square root of the acceleration voltage is shown as 20, and the excitation current of the second coil 3 is as shown with 20.
1, and the table is stored in the control device 4 in advance. Such a table of excitation currents for acceleration voltages can be easily obtained by computer simulation once the shape of the magnetic pole 1 of the objective lens and the configuration, arrangement, etc. of each coil are determined. In Fig. 2, A is the region where the chromatic aberration coefficient is dominant, B is the region where the spherical aberration coefficient and the chromatic aberration coefficient both contribute to the probe diameter, that is, the electron beam diameter, and C is the region where the spherical aberration coefficient is dominant. and a:
b indicates the ratio of excitation currents that minimizes the chromatic aberration coefficient, and C:
d indicates the ratio of excitation currents that minimizes the spherical aberration coefficient. In addition, in Fig. 2, a region B is considered in which both the spherical aberration coefficient and the chromatic aberration coefficient contribute to the probe current, and the ratio of the excitation currents of the first coil 2 and the second coil 3 is smoothly changed in this region B. However, this is to prevent problems such as axis misalignment or hysteresis effects that may occur if the ratio of excitation current is changed rapidly, making it difficult to use, and is essential. Basically, this region B does not need to be provided.

FIG. 3 shows an example of the magnetic field distribution on the optical axis 0 formed as described above. A solid line 25 shows the magnetic field distribution at high accelerating voltage, and a broken line 26 shows the magnetic field distribution at low accelerating voltage.

In this way, in the present invention, the excitation current supplied to the first coil 2 and the second coil 3 is changed to the excitation current supplied to the second coil 3 in order to minimize the chromatic aberration coefficient when the acceleration voltage is low. When the current is increased to obtain a narrow axial magnetic field distribution as shown by the broken line 26 in FIG. 3, and when the accelerating voltage is high,
In order to minimize the spherical aberration coefficient, the proportion of the excitation current supplied to the first coil 2 can be increased to obtain a wide axial distribution as shown by the solid line 25 in FIG. 3. In other words,
Conventionally, the objective lens had one coil, so when the excitation current was changed, the magnetic field strength changed, but the shape of the magnetic field distribution could not be changed.However, according to the present invention, at least two coils are used. By arranging two coils and changing the excitation current supplied to each coil according to the accelerating voltage, it is possible to obtain a magnetic field distribution that minimizes aberrations with respect to the accelerating voltage at that time. Regardless of the acceleration voltage, the spherical aberration coefficient and the chromatic aberration coefficient can be used in a small state, thereby improving the spatial resolution.

FIG. 4 is a diagram showing another example of the configuration of the objective lens, in which the magnetic pole 1' has one gap, but the first coil 2' is arranged outside the magnetic pole 1', and the first coil 2' is arranged inside the magnetic pole 1'. 2nd coil 3
' are arranged to generate lines of magnetic force indicated by 9' and 10', respectively, and form a predetermined magnetic field distribution. Moreover, FIG. 5 is a diagram showing still another example of the configuration of the objective lens, in which the magnetic pole 1
A first coil 2# and a second coil 3# are arranged side by side at #, and generate lines of magnetic force indicated by 9# and 10'', respectively, to form a predetermined magnetic field distribution. Although the control device, drive device, etc. are omitted in the figure,
As in the above embodiment, the control device stores a table in which the excitation current of each coil with respect to the acceleration voltage, which is determined in advance by simulation, is written.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications are possible. For example, in the above embodiment, the excitation current of each coil is determined only based on the accelerating voltage, but in addition, the excitation current of each coil may also be determined according to the working distance. Further, in the above embodiments, the case where the present invention is applied to an objective lens has been described, but it is clear to those skilled in the art that it is also possible to apply the present invention to other lenses, such as a condenser lens. Furthermore, it is clear to those skilled in the art that the magnetic pole shape and the excitation current table for acceleration voltage shown in the above embodiments are merely examples, and can be modified in various ways.

Furthermore, although two coils are arranged in the above embodiment, more coils may be arranged and the excitation current supplied to each coil may be determined by a table.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of an embodiment of the excitation method of an electromagnetic lens according to the present invention, Fig. 2 is a diagram showing an example of excitation current with respect to acceleration voltage, and Fig. 3 is a diagram showing the configuration of the first coil and the second coil. FIG. 4 is a diagram showing the configuration of another embodiment of the excitation method of the electromagnetic lens according to the present invention,
FIG. 5 is a diagram showing the configuration of still another embodiment of the excitation method for an electromagnetic lens according to the present invention. 1... Magnetic pole of objective lens, 2... First coil, 3...
... Second coil, 4... Control device, 5... First drive device, 6... 21st X moving device, 7... Acceleration voltage setting means, O... Optical axis. Applicant: JEOL Ltd.

Claims (1)

[Claims] (1) An electromagnetic lens in which at least two coils, a first coil and a second coil, are arranged; a first drive device that supplies an excitation current to the first coil; and an electromagnetic lens that supplies an excitation current to the second coil. 1. An excitation method for an electromagnetic lens, comprising: two drive devices and a control device, the control device determining an excitation current to be supplied to the first coil and the second coil in accordance with at least an acceleration voltage.