JPH0261093B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens for a scanning transmission electron microscope for observing a ferromagnetic sample.

Several objective lenses have been devised so far to observe images of ferromagnetic samples. All of these lenses have a hole for inserting the sample into the magnetic pole piece in order to shield the ferromagnetic sample from the lens magnetic field, but the first type of conventional objective lenses It generates a downward lens magnetic field to form an image of the electron beam that has passed through the sample.
Although this lens has the function of an imaging lens, it cannot form a lens magnetic field above the sample, so it does not have the function of narrowing down the electron beam irradiated onto the sample (control lens function). Also, the second
A lens of the type generates a magnetic field above the sample,
Although it had the function of narrowing the electron beam irradiated onto the sample, it was not possible to form an image of the electron beam that passed through the sample. Therefore, in the past, switching the observation mode of a ferromagnetic sample between a transmission image and a scanning image required replacing the objective lens or the magnetic pole piece. Furthermore, in conventional lenses, the electron beam transmitted through the sample is imaged on a transmission electron beam detector placed on or near the fluorescent plate. It is not possible to focus the electron beam irradiated onto the sample in a state where a diffracted electron beam of a specific order can be incident on the sample.
It was not possible to observe phase contrast images or dark field images of ferromagnetic samples in scanning transmission image mode.

The present invention solves these conventional drawbacks, and when observing a ferromagnetic sample, it is possible to switch the observation mode between a transmitted image and a scanned image without changing the objective lens or magnetic pole piece, and it is also possible to The purpose of this objective is to provide a magnetic field shielding objective lens for a scanning transmission electron microscope that allows phase contrast images or dark field images to be observed in a scanning transmission mode. first and second magnetic pole pieces disposed in the magnetic path of the magnetic flux via a first gap; and opposed to the second magnetic pole piece disposed in the magnetic path via a second gap. The distance between the third magnetic pole piece and the first magnetic pole piece is the second
a fourth magnetic pole piece provided so as to be smaller than the distance from the first magnetic pole piece; an auxiliary coil for passing a second magnetic flux between the fourth magnetic pole piece and the first magnetic pole piece; and the main coil. and a means for independently adjusting the excitation current of the auxiliary coil and the excitation current of the auxiliary coil, and configured so that a lens magnetic field in which the first magnetic flux and the second magnetic flux are superimposed is formed in the first gap. It is characterized by

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 is intended to show half of the cross section (with the optical axis as the boundary) of an embodiment of the present invention and the magnetic field strength along the optical axis in the lens gaps G1 and G2 of this embodiment. 1 is a main coil, and a lens current is supplied from a first lens power source 2 to this main coil. 3 is a yoke for guiding the magnetic flux generated from the main coil 1 to the magnetic pole piece. A first magnetic pole piece 4 is attached to the upper part of the yoke 3 (upper yoke part). A second magnetic pole piece 5 is arranged between the first magnetic pole piece 4 and a first gap G1. This magnetic pole piece 5 is supported by a spacer 6 made of a non-magnetic material. This second pole piece 5 is provided with a hole 8 into which a ferromagnetic sample 7 is inserted. The middle yoke part of yoke 3 has a second
A third magnetic pole piece 9 is attached opposite to the second magnetic pole piece 5 across the gap G2. Further, an auxiliary coil 10 is attached to the upper yoke portion, and an excitation current can be supplied to this auxiliary coil 10 from a second lens power source 11. The auxiliary coil is selected to have a magnetomotive force approximately 1/5 to 1/10 that of the main coil.
A fourth magnetic pole piece 12 is attached surrounding the auxiliary coil 10 and in contact with the upper yoke portion. The fourth magnetic pole 12 is arranged so that most of the magnetic flux generated by the auxiliary coil 10 passes between the first and fourth magnetic poles 4 and 12.
is attached at a position such that the distance from the first magnetic pole 4 is sufficiently smaller than the distance from the second magnetic pole 5.

Next, the operation will be explained with reference to FIG. 2, which shows the optical system of a scanning transmission electron microscope equipped with such an objective lens.

First, when attempting to observe a normal transmitted image, an excitation current is supplied from the lens power supply 2 to the main coil 1. As a result, a magnetic field as shown by the solid line A in FIG. 1 is generated in the first gap G1, and a magnetic field as shown by the solid line B in FIG. 1 is generated in the second gap G2, completely shielding the sample 7. A lens magnetic field is formed in this state. As a result, Figure 2a
As shown in FIG. 2, the electron beam EB enters the sample 7 without being narrowed down by the front magnetic field lens 13a formed by the magnetic field in the gap G1. Therefore, the electron beam transmitted through the sample 7 is connected to the object surface of the intermediate lens 14 as a transmitted image T1 by the rear magnetic field lens 13b of the objective lens formed by the magnetic field of the gap G2, and therefore is projected onto the fluorescent screen 16. A transmission image T3 of the sample 7 is formed.
Note that 15 is a projection lens, and D1 is a diffraction image of the electron beam transmitted through the sample 7.

Furthermore, when trying to obtain a diffraction image of the sample 7, by aligning the object plane of the intermediate lens 14 with the position of the diffraction image D1 while keeping each lens in the excited state shown in FIG. 2a, the image shown in FIG. As shown in b, a diffraction image is projected on the fluorescent screen 16.

Therefore, when attempting to observe the phase contrast image of the sample 7 using a scanning transmission image, the intermediate lens 14
An excitation current is supplied to the second lens power supply 11 shown in FIG. 1 while maintaining the excitation in the state shown in FIG. 2b. As a result, a second magnetic flux is generated from the auxiliary coil 10, but this magnetic flux mainly passes between the first and fourth magnetic pole pieces 4 and 12. Therefore, the second gap G
The magnetic field strength at 2 remains the same as shown by the solid line B in FIG. The magnetic field strength of G1 changes to that shown by the dotted line C in FIG. As a result, the electron beam EB is narrowly focused onto the surface of the sample 7 by the front magnetic field lens 13a, as shown in FIG. 2c. In this case, since the lens magnetic fields of other lenses including the rear magnetic field lens 13b do not change, the sample 7 on the fluorescent screen 16 does not change.
A diffraction image of the transmitted electron beam is subsequently obtained. Therefore, the fluorescent screen 16 is removed, and the first-order diffracted electron beam of this diffraction image is made incident on electron beam detectors 17a and 17b, which are normally arranged slightly below the fluorescent screen 16. Therefore, the electron beam incident on sample 7 is
If the EB is scanned by a deflector not shown, and the output signals of the detectors 17a and/or 17b at that time are supplied to the CRT being scanned in synchronization with this scanning, a dark-field image in the scanning transmission mode is displayed on the CRT. can be obtained. Moreover, the detectors 17a and 17
By supplying the difference signal of b to the CRT, a phase contrast image in scanning transmission mode can be obtained.

As is clear from the above description, in the magnetically shielded objective lens according to the present invention, only the strength of the front magnetic field lens of the objective lens can be changed without substantially changing the strength of the rear magnetic field lens of the objective lens. Therefore, the degree of focusing of the electron beam irradiated onto the sample can be adjusted by changing the intensity of the front magnetic field lens while maintaining the excitation state in which a transmission image is formed based on the electron beam transmitted through the sample by the rear magnetic field lens. can be adjusted freely,
Therefore, the observation mode of the sample can be switched between a transmission image and a phase contrast image (or dark field image) in the transmission scanning image mode without replacing the objective lens or the magnetic pole pieces of the objective lens.

[Brief explanation of the drawing]

FIG. 1 shows a cross section of an embodiment of a magnetically shielded objective lens according to the present invention, and this lens gap G1,
FIG. 2 is a diagram illustrating the strength of the magnetic field generated in G2, and is a diagram illustrating the optical system during various image observations in a scanning transmission electron microscope using a magnetic field shield objective lens based on the present invention. 1, 10: Excitation coil, 2, 11: Lens power supply, 3: Yoke, 4, 5, 9, 12: Magnetic pole piece,
6: Spacer, 7: Sample, 8: Hole, G1, G
2: Gap, 13a: Front magnetic field lens, 13
b: Back magnetic field lens, 14: Intermediate lens, 15:
Objective lens, 16: Fluorescent screen, 17a, 17b: Electron beam detector, T1, T2, T2: Transmission image, D1,
D2: Diffraction image.

Claims (1)

[Claims] 1 A main coil, first and second magnetic pole pieces disposed through a first gap in a magnetic path of a first magnetic flux generated from the main coil, and a second gap in the magnetic path. The distance between the first magnetic pole piece and the third magnetic pole piece, which is disposed opposite to the second magnetic pole piece through the
a fourth magnetic pole piece provided so as to be smaller than the distance from the first magnetic pole piece; an auxiliary coil for passing a second magnetic flux between the fourth magnetic pole piece and the first magnetic pole piece; and the main coil. and a means for independently adjusting the excitation current of the auxiliary coil and the excitation current of the auxiliary coil, and configured so that a lens magnetic field in which the first magnetic flux and the second magnetic flux are superimposed is formed in the first gap. A magnetically shielded objective lens featuring