JPH04324239A - Objective lens in electron microscope - Google Patents

Abstract

PURPOSE:To provide an objective lens of an electron microscope enabling observation under a condition where a sample is laid at all times in a non-electric field. CONSTITUTION:A current shifted from a positive focus current I is supplied to a main coil current supply circuit 9 from a main coil current designation circuit 8. The signal representing a value of the current is supplied to a compensation coil current supply circuit 10. When the circuit 10 receives the current signal, it controls the current to be supplied to a compensation coil 5 according to the signal. As a result, in the vicinity of a sample 3 between a magnetic pole pieces 4a and 4c, magnetic flux capable of cancels the leak magnetic flux in the vicinity of the sample 3 is generated, and thus the sample 3 can be observed while the sample 3 is laid under non-magnetic condition at all times.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an electron microscope that shields a magnetic sample from a lens magnetic field.

0002

2. Description of the Related Art FIG. 3 shows an objective lens of a conventional transmission electron microscope. In the figure, 1 is a yoke;
2 is the main coil, 3 is the sample, 4a, 4b, 4c are the magnetic pole pieces,
5 is a correction coil, 6 is a main coil current supply circuit, and 7 is a correction coil current supply circuit.

In the objective lens of such an electron microscope, a current I1 necessary for forming a sample image on a fluorescent screen (not shown) is supplied to the main coil 2 from the main coil current supply circuit 6. Since the distance between the sample 3 and the fluorescent screen is fixed, the current I1 has a specific value depending on this distance. By supplying this current, the yoke 1, the magnetic pole piece 4
A magnetic flux with magnetic paths a and 4b is generated. Then, leakage magnetic flux in the direction shown in FIG. 3 is generated around the sample 3. This leakage magnetic flux is generated because the magnetic resistance near the tip of the magnetic pole piece 4a is large. Due to this leakage magnetic flux, when the sample is a ferromagnetic material such as steel, beam shift, tilt, and astigmatism may occur in electron beam irradiation, and image deviation and optical axis deviation may occur in imaging. This causes blurring of the image due to misalignment and astigmatism. Therefore, a current I2 for canceling out the leakage magnetic flux is supplied from the correction coil current supply circuit 7 to the correction coil 5. This current I2
is a constant current determined experimentally in advance. By supplying this current, magnetic flux necessary to cancel out the leakage magnetic flux is generated between the magnetic pole pieces 4a and 4c, and the vicinity of the sample becomes free of magnetic field. As a result, even if the sample is a ferromagnetic material, the above-mentioned problems do not occur, and a high-quality image can be observed.

0004

[Problem to be solved by the invention] Now, the sample 3
When observing magnetic domains, the focus of the sample image is moved slightly away from the positive focus (Lorentz microscopy). This operation can be done by shifting the current flowing through the main coil 2 from the current I1 at the positive focus. However, by shifting the current I1, the intensity of the leakage magnetic flux also changes. Therefore, even if a constant current I2 is supplied to the correction coil 5 as described above,
The magnetic flux that cancels out the leakage magnetic flux is applied to the magnetic pole pieces 4a and 4.
It cannot occur between c.

The present invention has been developed in view of these points, and it is possible to observe the sample while always placing it in a non-magnetic field even if the excitation intensity of the objective lens is changed according to various requirements. The aim is to create an objective lens for an electron microscope that makes it possible.

[0006]

[Means for solving the problem] To that end, the present invention
first and second magnetic pole pieces, a main coil for forming a lens magnetic field between the first and second magnetic pole pieces, and a sample disposed outside between the first and second magnetic pole pieces. , a thin electron beam passage hole formed in the first magnetic pole piece to suppress leakage of the lens magnetic field to the sample side through the electron beam passage hole;
In order to generate a magnetic flux in the opposite direction to the magnetic flux leaking from the first magnetic pole piece to the vicinity of the sample, a third magnetic pole piece is provided to sandwich the sample and face the first magnetic pole piece, and a third magnetic pole piece is provided in the opposite direction. a correction coil for generating a magnetic flux; and an excitation current supplied to the correction coil in accordance with an excitation current supplied to the main coil so that the magnetic flux in the opposite direction cancels out the leakage magnetic flux near the sample. The present invention features an electron microscope objective having means for controlling the electron microscope objective.

[0007]

EXAMPLE FIG. 2 shows the relationship between the current I flowing through the main coil 2 and the leakage magnetic flux density B around the sample 3 after repeated experiments by the present inventor several times. The density B of the leakage magnetic flux is expressed as a function B=f(I) of the current I.

FIG. 1 shows an objective lens of a transmission electron microscope shown as an embodiment of the present invention. In the figure, the same numbers as in FIG. 3 indicate the same components. 8
9 is the main coil current specification circuit, 9 is the main coil current supply circuit,
10 is a correction coil current supply circuit. The correction coil 5 generates a magnetic flux in the opposite direction to the leakage magnetic flux between the magnetic pole pieces 4a and 4c. Here, the correction coil current supply circuit 10 is a circuit for supplying the correction coil 5 with a current necessary for the magnetic flux in the opposite direction to cancel out the leakage magnetic flux near the sample. The correction coil current supply circuit 10 has a built-in table that stores correction coil current designation data created based on the function. Now, when observing the magnetic domain of the sample 3, the main coil current designation circuit 8 supplies the main coil current supply circuit 9 with a designation signal representing a current that deviates from the current I1 at the positive focus. Then, a current based on the signal is supplied from the main coil current supply circuit 9 to the main coil 2. Further, the signal is sent to the correction coil current supply circuit 10. When the correction coil current supply circuit 10 receives the current signal, it specifies the address of the table based on the current signal and reads out the data at the specified address. The correction coil current supply circuit 10 supplies a predetermined excitation current to the correction coil 5 based on this data. As a result, magnetic flux is generated between the magnetic pole pieces 4a and 4c just enough to cancel out the leakage magnetic flux near the sample in this case. Therefore, in cases such as observing the magnetic domains of a sample, even if the excitation intensity of the objective lens is changed from the normal case, the sample can always be observed in the absence of a magnetic field. Therefore, high-quality images can always be observed even with magnetic samples.

In the above-described embodiment, a table storing correction coil current designation data is used in order to control the current supplied to the correction coil in accordance with the current supplied to the main coil. A program for determining the current to be supplied to the correction coil from an arithmetic expression corresponding to B=f(I) may be stored, and the correction current may be controlled by this program.

Furthermore, the present invention can be similarly applied to the case where the excitation current of the objective lens is changed depending on, for example, the accelerating voltage.

[0011]

[Effects of the Invention] The present invention provides first and second magnetic pole pieces;
A main coil for forming a lens magnetic field between the first and second magnetic pole pieces, a sample placed outside between the first and second magnetic pole pieces, and a main coil for forming a lens magnetic field between the first and second magnetic pole pieces, and a main coil for forming a lens magnetic field between the first and second magnetic pole pieces. A thin electron beam passing hole is formed in the first magnetic pole piece to suppress leakage to the sample side through the electron beam, and a magnetic flux is generated in the opposite direction to the magnetic flux leaking from the first magnetic pole piece to the vicinity of the sample. A third magnetic pole piece is provided to sandwich the sample and face the first magnetic pole piece, and a correction coil is provided to generate the magnetic flux in the opposite direction, and the magnetic flux in the opposite direction is connected to the leakage magnetic flux. Since the device is provided with means for controlling the excitation current supplied to the correction coil in accordance with the excitation current supplied to the main coil so as to cancel each other in the vicinity of the sample, the first a correction coil for generating a magnetic flux in the opposite direction to the magnetic flux leaking from the magnetic pole piece to the vicinity of the sample between the second gaps, and a magnetic flux generated based on the excitation of the correction coil that is the leakage magnetic flux. Since the device is equipped with a means for controlling the excitation current supplied to the correction coil according to the excitation current supplied to the main coil so as to cancel each other in the vicinity of the sample, when the excitation current of the objective lens is variously changed, However, by always placing the sample in a non-magnetic field, it is possible to observe high-quality images without the influence of magnetization.

[Brief explanation of the drawing]

FIG. 1 shows an objective lens of a transmission electron microscope shown as an embodiment of the present invention.

FIG. 2 shows the relationship between the current flowing through the main coil and the leakage magnetic flux generated around the sample.

FIG. 3 shows an objective lens of a conventional transmission electron microscope.

[Explanation of symbols]

1... Yoke, 2... Main coil, 3... Sample, 4a, 4b, 4
c... Magnetic pole piece, 5... Correction coil, 6... Main coil current supply circuit, 7... Correction coil current supply circuit, 8... Main coil current specification circuit, 9... Main coil current supply circuit, 10... Correction coil current supply circuit

Claims (1)

[Claims] Claim 1: First and second magnetic pole pieces;
a main coil for forming a lens magnetic field between the magnetic pole pieces of the
The sample is placed outside between the first and second magnetic pole pieces, and the lens is formed on the first magnetic pole piece in order to prevent the magnetic field from leaking to the sample side through the electron beam passage hole. a thin electron beam passage hole, and a third magnetic pole piece provided opposite to the first magnetic pole piece to sandwich the sample in order to generate a magnetic flux in the opposite direction to the magnetic flux leaking from the first magnetic pole piece to the vicinity of the sample. a magnetic pole piece, a correction coil for generating the magnetic flux in the opposite direction, and an excitation current supplied to the correction coil to the main coil so that the magnetic flux in the opposite direction cancels out the leakage magnetic flux in the vicinity of the sample. An objective lens for an electron microscope equipped with means for controlling according to an excitation current.