JPH06223767A - Direct imaging type reflection electron microscope - Google Patents

Abstract

PURPOSE:To provide a direct imaging type reflection electron microscope whose an irradiation optical system and an expanded image forming optical system are arranged roughly in parallel with aberration lessened between an incident beam and a formed image. CONSTITUTION:Let a beam separator 3 consist of three section type magnets 31 through 33, the second sector type magnet 32 has three end faces, one end face is faced to a specimen S, and the other two end faces are faced to the first and the second sector type magnet 31 and 33 respectively so as to be disposed. And an irradiation system 2 is connected to the first sector type magnet 31, and an image forming system 5 is connected to the third sector type magnet 33, so that the irradiation system 2 and the image forming system 5 are disposed almost symmetrically about the line A normal to the surface of the specimen S. By this constitution, the aberration between each incident beam on the specimen S and a formed image on a screen 6 can be lessened, so that an excellent reflection type electron image can be obtained, and adjustment such as axial alignment can concurrently be made with ease. Moreover, the degree of freedom in arrangement in an optical system can also be made high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection electron microscope, and more particularly to a beam separator for separating an incident electron beam on a sample and a reflected electron beam from the sample, so that an irradiation optical system and a magnifying imaging optical system can be obtained. The present invention relates to a direct mapping type reflection electron microscope in which the system is made parallel and aberration is reduced.

[0002]

2. Description of the Related Art Electron microscopes are classified into a transmission type in which electrons are transmitted through a sample and a reflection type in which electrons are reflected. There are a direct mapping type and a scanning type from the viewpoint of image formation. From these two viewpoints, the transmission type and direct mapping type are electron microscopes called TEM, and the transmission type and scanning type are electron microscopes called STEM. A reflective type and a scanning type are SEMs. On the other hand, the reflection type and the direct mapping type are LEEM or LEERM (Low Energy Ref
lecgtion Microscope).

In LEEM, low-acceleration electrons of 20 to 200 V are applied perpendicularly to the sample surface, and an electron is directly obtained by using the electrons reflected from the sample surface. The principle of image formation is the same as that of a TEM (transmission electron microscope), but since an image is obtained using reflected electrons, an optical system that is bent with respect to the sample surface is required. FIG. 3 is a diagram schematically showing an optical system of a conventional LEEM, in which an electron beam from an electron gun 1 arranged obliquely above the surface of the sample S is focused by an irradiation lens system 2 so that the sample surface is irradiated with the electron beam. It is irradiated so that it is incident at an angle. This oblique electron beam is bent perpendicularly to the sample surface by the beam separator 3, decelerated by the cathode objective lens 4 and vertically incident on the sample S. Then, the electrons reflected from the sample S are accelerated by the cathode objective lens 4, enter the beam separator 3, are bent to the opposite side this time, are separated obliquely upward from the incident electron beam, and form the imaging lens system 5. An image is formed on the screen 6 by. Sample S
Since the velocity of electrons is low in the vicinity and is easily affected by disturbance, it is usually possible to accelerate the cathode objective lens 4 immediately after it comes out of the sample S and pass through the optical system at an acceleration voltage of 10 to 20 kV as described above. Done. Similarly, the incident beam also passes through the irradiation optical systems 2 and 3 at an acceleration voltage of 10 to 20 kV and is decelerated by the cathode objective lens 4 immediately before the sample S.

As described above, the cathode objective lens 4 has a role of accelerating and decelerating electrons and, at the same time, enlarging an image as a first stage lens.

Further, since the irradiation beam and the reflected beam pass through the same path in the vicinity of the sample S, it is necessary to separate the paths of the irradiation beam and the reflected beam. The beam separator 3 plays this role, and a magnetic field type sector is usually used. Fortunately, one magnetic field type sector can be used for this allocation. This is because the Lorentz force exerts a force in the same direction as the traveling direction, but since the electrons have opposite directions, they are bent in the opposite direction.

However, the secondary aberration created by the magnetic field type sector distorts (or blurs) the finally obtained image. As a measure to minimize the effect of aberration, the angle between the incident beam and the reflected beam is made as small as possible, or a Wien filter is used, and only the reflected beam is emitted perpendicular to the sample. Avoid the effects of secondary aberrations (Japanese Patent Application No. 3-
199134) etc. have been devised.

[0007]

[Problems to be Solved by the Invention]
The effect of the incident beam on the image formed has not been taken into account very much. Considering the analogy with TEM, it must be considered that the influence of the angle of the incident beam on the image is extremely large. In particular, in alignment of axes, it is necessary that the pattern of the incident beam does not include aberration or axis deviation.

The present invention has been made in view of such a situation, and an object thereof is to arrange an irradiation optical system and a magnifying image forming optical system substantially in parallel with each other to reduce aberrations of an incident beam and an image formed image. Another object of the present invention is to provide a direct mapping reflection type electron microscope.

[0009]

The direct mapping reflection electron microscope of the present invention which achieves the above object comprises an electron beam with which a sample is irradiated and an electron beam which is reflected from the sample and travels in the opposite direction to the irradiation electron beam. In a direct mapping reflection electron microscope which separates by a beam separator, the beam separator is composed of three sector type magnets, the second sector type magnet has three end faces, and one end face faces the sample,
The other two end faces are respectively arranged to face the first and third sector type magnets, the irradiation system is connected to the first sector type magnet, and the imaging system is connected to the third sector type magnet.
The irradiation system and the imaging system are arranged substantially symmetrically with respect to the normal to the sample surface.

[0010]

In the present invention, the beam separator is composed of three sector type magnets, and the second sector type magnet is three.
Two end faces, one end face facing the sample, and the other two end faces facing the first and third sector magnets, respectively, and the irradiation system connected to the first sector magnet. An image forming system is connected to the third sector magnet, and the irradiation system and the image forming system are arranged substantially symmetrically with respect to the normal to the sample surface. Aberrations can be reduced, a good backscattered electron image can be obtained, and adjustments such as axis alignment become easy. Moreover, the degree of freedom in the arrangement of the optical system is increased, and the device can be made compact.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the direct image reflection type electron microscope of the present invention will be described below with reference to the drawings. The present invention is characterized by a beam separator in consideration of the aberration produced by an incident beam. Is.

FIG. 1 is a diagram showing a schematic structure of an LEEM according to the present invention. This apparatus has an optical axis of an irradiation optical system composed of an electron gun 1 and an irradiation lens system 2, an imaging lens system 5 and a screen. 6 is parallel to the optical axis of the magnifying and imaging optical system, and the sample S is arranged on an axis A intermediate between the optical axes. Therefore, the beam separator 3 has three sector magnets. It is composed of 31, 32, and 33.
With this arrangement, the electron beam from the electron gun 1
The irradiation lens system 2 squeezes the light in parallel with the axis A, bends it to the right by the first sector magnet 31, enters the second sector magnet 32, and the second sector magnet 32 in the same direction in the opposite direction. The sample is bent at an angle and emitted along the axis A, is decelerated by the cathode objective lens 4, and is vertically incident on the sample S. Then, the electrons reflected from the sample S are
Accelerated by the cathode objective lens 4, this time it is separated from the incident beam in the opposite direction by the second sector type magnet 32 and enters the third sector type magnet 33, and in the opposite direction by the third sector type magnet 33. It is bent at substantially the same angle, is emitted along an axis parallel to the axis A, and is imaged on the screen 6 by the imaging lens system 5.

Here, each sector magnet 31, 32, 3
The end surface of 3 may be, of course, provided with a taper or a curved surface in order to form an image without astigmatism and to remove the second-order aberration, like an ordinary mass spectrometer or an omega filter for an electron beam. In FIG. 1, these end faces are not shown for simplicity. Also, a mirror for adjusting the fringe field may be provided.

These three sector type magnets 31, 32, 3
Considering the sample S as a mirror, 3 is the two sector type magnets 32, 33 in the latter half of the omega filter composed of four sector type magnets 31, 32, 32, 33 as shown in FIG. It can be understood that it corresponds to the one rotated by 180 °. That is, the beam separator 3 composed of the three sector magnets 31, 32, and 33 corresponds to an omega filter in terms of electron optics, and has a first-order aberration, a second-order aperture aberration, and a curvature aberration at the center symmetry plane. It is possible to realize most of the optical characteristics of the omega filter, such as eliminating the noise. Thus, for example, the first
The second-order aberration caused by the sector type magnet 31 can be corrected by the second sector type magnet 32.
The same applies to the relationship between the sector type magnet 32 and the third sector type magnet 33. Therefore, the aberration of the incident beam on the sample S and the image formed on the screen 6 can be reduced. Also, the energy spread of the incident beam is
It is also possible to set the parameters of each sector magnet so that dispersion does not occur at the outlet of the third sector magnet 33, and the spread of energy produced by the sample S is dispersed at the outlet of the third sector magnet 33. And can also act as an energy analyzer. Further, in the second sector type magnet 32, it is possible to make the angle formed by the beams on the incident side and the emission side as very large as θ≈80 ° to 110 °, and in the conventional beam separator, θ≈
It is very large as compared with 15 ° to 45 °, and the problem of contact between the optical system in the preceding stage and the optical system in the subsequent stage is less likely to occur, and these optical systems can be arranged close to each other.

Although the direct mapping reflection type electron microscope of the present invention has been described based on the embodiment, the present invention is not limited to this embodiment and various modifications can be made.

[0016]

As is apparent from the above description, according to the direct mapping reflection type electron microscope of the present invention, the beam separator is composed of three sector type magnets, and the second sector type magnet has three end faces. One of the end faces faces the sample, and the other two end faces are arranged to face the first and third sector magnets, respectively, and the irradiation system is connected to the first sector magnet. Since the imaging system is connected to the sector magnet and the irradiation system and the imaging system are arranged almost symmetrically with respect to the normal to the sample surface, the incident beam on the sample and the aberration of the image formed on the screen are reduced. Therefore, a good backscattered electron image can be obtained, and adjustments such as axis alignment become easy. Moreover, the degree of freedom in the arrangement of the optical system is increased, and the device can be made compact.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an LEEM according to the present invention.

FIG. 2 is a diagram for explaining that the LEEM according to the present invention corresponds to an omega filter.

FIG. 3 is a diagram showing a schematic configuration of a conventional LEEM.

[Explanation of symbols]

 S ... Sample A ... Central axis 1 ... Electron gun 2 ... Irradiation lens system 3 ... Beam separator 4 ... Cathode objective lens 5 ... Imaging lens system 6 ... Screen 31 ... First sector magnet 32 ... Second sector magnet 33 ... 3rd sector type magnet 32 ... sector type magnet 32 ... sector type magnet

Claims (1)

[Claims] 1. A direct mapping type reflection electron microscope in which an electron beam irradiating a sample and an electron beam reflected from the sample and traveling in a direction opposite to the irradiating electron beam are separated by a beam separator, and the sector has three beam separators. The second sector magnet has three end surfaces, one end surface faces the sample, and the other two end surfaces face the first and third sector magnets, respectively. The irradiation system is connected to the first sector magnet, the imaging system is connected to the third sector magnet, and the irradiation system and the imaging system are arranged substantially symmetrically with respect to the normal to the sample surface. Direct mapping type backscattered electron microscope characterized by the following.