JP2966438B2 - Scanning interference electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to the addition of a new function to a scanning electron microscope used for observing the structure of a sample surface and evaluating the crystallinity of a minute area, and particularly relates to a new signal detection. The present invention relates to a microscope aiming at improving detection sensitivity and enabling quantitative evaluation by a method.

[Conventional technology]

FIG. 6 shows a basic configuration of a conventional scanning electron microscope. This is discussed in Japanese Journal of Applied Physics, Vol. 23 (1984), pp. 913-920 (Japan J. Appl. Phys., 23 (1984) pp913-p.
p920). FIG. 6 operates as follows.

A primary electron beam 3 emitted from an electron gun 2 having an acceleration power supply 1 is focused by a focusing lens 4 on a surface of a sample 7 in a vacuum vessel 6. The primary electron beam deflection coil group 5 is operated by the scanning power supply 18 to scan the primary electron beam 3 on the surface of the sample 7. At this time, the signal of the secondary electrons 9 emitted from the surface of the sample 7 is replaced by the secondary electron detector 8 with a luminance modulation signal of a cathode ray tube (hereinafter abbreviated as CRT) 17 on the CRT 17. Obtain a secondary electron image. With this secondary electron image, an uneven structure on the order of several tens of nm on the sample 7 can be observed. Also,
The electron beam 10 reflected or transmitted from the sample 7 is a fluorescent plate.
It is observed through a viewing window 12 as a diffraction image on 11. By analyzing this diffraction image, it becomes possible to analyze the crystal state (arrangement state of elements constituting the surface and inside of the sample 7) at an arbitrary position of the sample 7. In addition, optical lenses
13. Select a specific diffraction spot using the aperture 14 and the optical fiber 15, and synchronize the electric signal obtained from the photoelectric conversion element (for example, a photomultiplier) 16 with the scanning of the primary electron beam 3 to modulate the brightness of the CRT 17. By converting into a signal, a diffraction microscope image is obtained on the CRT 17.

Not only the crystal distribution of the sample 7 can be known from the diffraction microscope image, but also the intensity change of the reflected or transmitted electron beam 10 is sensitive to slight changes in the crystal structure and orientation on the sample surface. The structure at the upper atomic layer level can be observed.

Further, Japanese Patent Application Laid-Open No. 64-65762 discloses that an electron beam is separated by a biprism and interfered without narrowing the electron beam to obtain an electron beam hologram image. However, this is for realizing electron beam holography by a transmission electron microscope, and has a completely different object from the present invention.

[Problems to be solved by the invention]

As described above, the conventional scanning electron microscope is a powerful device for observing the microcrystalline state of the sample 7 and the fine structure at the atomic layer level on the surface of the sample 7. However, the conventional apparatus has the following problems. That is, it is difficult to quantitatively evaluate the observed unevenness structure at the atomic layer level, and it is difficult to observe the uneven structure itself because the contrast decreases when the change in the crystal structure and orientation associated with the uneven structure at the atomic layer level is small. There was a problem of becoming.

An object of the present invention is to solve the above problems in the prior art,
An object of the present invention is to provide a scanning electron microscope capable of quantitatively evaluating an uneven structure at an atomic layer level and obtaining a clear scanning electron microscope image.

[Means for Solving the Problems] In order to achieve the above object, a scanning interference electron microscope according to the present invention comprises an electron source, a separating means for separating an electron beam from the electron source, and a separating electron beam. A focusing lens for narrowing down, a deflecting means for scanning the sample with the separated and focused electron beam, a sample stage on which the sample is placed, and a sample stage arranged in a direction in which the electron beam passes through the sample stage. Detecting means for detecting the formed image.

[Operation] The operation will be described by taking as an example a case where there is a slight step on the surface of the sample and the size is quantitatively known.

It is assumed that the electron beam separated into two by the biprism is focused and scanned on the sample, and a reflected wave from the sample is used. When one electron beam is above the step on the sample and the other is below, a phase difference corresponding to the height of the step occurs between two reflected waves from the sample. Interference between these two electron beams causes interference fringes. The position of the interference fringes changes by the phase difference caused by the step compared to the case where there is no step on the sample. Since the amount of change corresponds to the size of the step on a one-to-one basis, the size of the step can be quantitatively known from the amount of change. Further, when detecting the amount of change from the position of the interference fringes, a scanning electron microscope image is obtained by using only the intensity of the interference fringes in a specific positional relationship, such as the position of the highest intensity of the interference fringes, as a signal. This makes it possible to clearly observe the steps.

The same principle is applied to a case where an electron beam is transmitted through a sample and the phase of one electron beam is changed with respect to the other electron beam.

When the electron beam is reflected on a sample having a step as described above and reflected, and then separated by a biprism and interferes, the electron beam incident on the sample is overfocused or underfocused. If the beam is incident in a state of defocus, a phase difference corresponding to the size of the step is generated between the portions of the electron beam reflected from the upper and lower portions of the step among the electron beams reflected at the step of the sample. Therefore, the same effect as described above can be obtained by separating the electron beams so that the electron beams corresponding to these portions are obtained by the biprism and causing them to interfere with each other.

The same applies to the case where an overfocused or underfocused electron beam is transmitted through a sample to give a phase difference between portions of the electron beam.

That is, as in the above example, a biprism capable of dividing one electron beam into two electron beams is provided with an electron beam path, and the two electron beams separated by the biprism are focused and incident on a sample, Alternatively, the defocusing of one electron beam on the sample and the incidence on the sample are performed to cause a phase difference between the electron beams or the portions of the electron beam obtained by reflection or transmission from the sample. is there. Therefore, a phase difference corresponding to a phase change due to the sample structure can be generated between electron beams obtained through the biprism and the sample. By obtaining signals for microscopic images based on the interference intensity of these electron beams, it becomes possible to quantitatively evaluate the concavo-convex structure at the atomic layer level, and at that time obtain a clear electron microscopic image as described above. It becomes possible.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

In FIG. 1, a primary electron beam 3 emitted from an electron gun 2 is converged by a converging lens 4 after passing through a biprism composed of a fine wire 19 to which a positive voltage is applied and a grounded metal plate 20. The primary electron beam 3 is equivalently split into two by the action of the biprism, and the split primary electron beam 3 is focused by the focusing lens 4 at two points on the sample 7. The reflected or transmitted electron beam 10 emitted from these two points interferes at point B where both electron beams overlap, and forms interference fringes. The whole image formed by the reflected or transmitted electron beam 10 is observed on the fluorescent plate 11, and a specific reflected or transmitted electron beam 10 is selected by the reflection or transmitted electron beam deflection coil group 21. The interference fringes formed by the reflected or transmitted electron beam 10 are enlarged by a magnifying lens 22 to form interference fringes on a flat secondary electron multiplier 23 (for example, a channel plate). The intensity of this interference fringe is amplified by the flat secondary electron multiplier 23, and the interference fringe is observed as an optical signal on the fluorescent plate 24 in the latter stage of the flat secondary electron multiplier. A real image of the interference fringes is formed on the image sensor 25 (for example, a CCD device) by the optical lens 13 through the viewing window 12, and the pixel signals of the image sensor 25 are stored in the memory of the computer. At this time, as shown in FIG. 2 (a), only the pixel signal at the place where the interference fringe exists is masked by the internal processing of the computer 26 and used as the luminance modulation signal of the CRT 17. A scanning electron microscope image can be obtained by operating the primary electron beam deflection coil group 5 by the scanning power supply 18 and scanning the primary electron beam 3 on the sample 7. This scanning electron microscope image becomes a sample distribution image that gives a specific interference fringe. Now, as shown in FIG. 3 (a), when the separated primary electron beam 3 is focused on both sides of the step existing on the sample 7, the height of the step is d and the wavelength of the primary electron beam 3 is Assuming that λ and the incident angle of the primary electron beam 3 to the sample 7 are θ, 2π · 2dsin
A phase difference of θ / λ occurs. The acceleration voltage of the primary electron beam 3 is set to 15
If kV, the height d of the step is 1.4 ° (corresponding to a monoatomic layer step on the Si (001) plane), and the incident angle θ is 3 °, the phase difference is 2π × 1.5. Since the interval between the interference fringes shown in FIG. 2A corresponds to a phase difference of 2π, a shift of the interference fringes occurs at the location of the step as shown in FIG. 2B. From the value of the shift, the height d of the step can be quantitatively known. Further, at this time, since the interference fringes are masked by the presence of the step, the step becomes a dark image on the CRT 17, so that the step image can be observed as a clear contrast image. Also, as shown in FIG.
When only one of the two primary electron beams 3 is incident on the sample 7, depending on the phase difference between the other electron beam and the electron beam resulting from the incidence, FIG. A similar effect can be obtained. Further, by setting the potential applied from the voltage applying power supply 27 to the ultrafine wire 19 to the ground potential, it is possible to make it equivalent to the conventional scanning electron microscope shown in FIG.

FIG. 4 is a reference diagram, and a very thin wire 19 to which a positive voltage is applied.
This is an embodiment showing a method of obtaining an interference fringe by separating a reflected or transmitted electron beam 10 into two beams and making them interfere with each other by a biprism composed of a metal plate 20 grounded and grounded. The interference fringes obtained by separating and interfering the electron beam 10 reflected or transmitted from the sample 7 into two by the biprism are enlarged by the magnifying lens 22 to form interference fringes on the planar secondary electron multiplier 23. Configuration. Other configurations and operations are the same as those of the embodiment shown in FIG. When the primary electron beam 3 is scanned on the sample 7, the focusing of the primary electron beam 3 is shifted (overfocus or underfocus) as shown in FIG. At this time, when there is a step on the sample 7, 2π × between the hatched portion and the non-hatched portion of the reflected or transmitted electron beam 10 as in FIG.
A phase difference of 2dsin θ / λ occurs. When the portion where the phase difference occurs is separated by the biprism and interferes, the position of the interference fringes is shifted by a value corresponding to the phase difference. From the value of the shift, the height d of the step can be quantitatively known. Further, at this time, the interference fringes whose positions are shifted by the phase difference are masked, so that the steps become dark images on the CRT 17, and as a result, the step images are observed as clear contrast images. Also, as shown in FIG. 5 (b), when only the hatched part of the primary electron beam 3 is incident on the sample 7, the reflected or transmitted electron beam 10 resulting from the incidence is mixed with the hatched part. Depending on the phase difference
It is possible to obtain the same effect as that of FIG.

〔The invention's effect〕

As described above, the scanning electron microscope according to the present invention is:
In order to use the displacement of the interference fringes resulting from the phase difference of the electron beam as the detection signal, it is possible not only to quantitatively know the height of the step at the atomic layer level on the sample that generates the phase difference, but also to obtain the crystal near the step. It has an extremely excellent advantage that the uneven structure which is difficult to observe with a conventional scanning electron microscope can be clearly observed because the change in the structure and orientation is small.

[Brief description of the drawings]

FIG. 1 is a structural view of an embodiment in which a biprism is provided between an electron gun and a sample and an electron beam from the electron gun is separated into two by a biprism and interfered with each other. FIG. FIG. 3 is a detailed view of the portion indicated by A in FIG. 1,
FIG. 4 is a reference view of a reference embodiment in which a biprism is provided after the sample and reflected or transmitted electron beams from the sample are separated into two by the biprism and interfered with each other. FIG.
FIG. 6 is a block diagram of a conventional device. DESCRIPTION OF SYMBOLS 1 ... Acceleration power supply 2 ... Electron gun 3 ... Primary electron beam 4 ... Focusing lens 5 ... Deflection coil group for primary electron beam 6 ... Vacuum container, 7 ... Sample 8 ... Secondary electron detection Vessel 9 secondary electron 10 reflected or transmitted electron beam 11 fluorescent plate 12 viewing window 13 optical lens 14 aperture 15 optical fiber 16 photoelectric conversion element 17 ... Cathode tube 18 Scanning power supply 19 Ultrafine wire 20 Metal plate 21 Deflection coil group for reflective or transmitted electron beam 22 Magnifying lens 23 Planar secondary electron multiplier 24 … Flat plate secondary electron multiplier tube post-stage fluorescent plate 25 …… Image sensor 26 …… Calculator 27 …… Voltage application power supply

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01J 37/28-37/295 H01J 37/22 H01J 37/256 G01B 15/00

Claims (3)

(57) [Claims] An electron source; a separating means for separating an electron beam from the electron source; a focusing lens for narrowing the separated electron beam; and a deflecting means for scanning a sample with the separated and focused electron beam. A scanning stage electron microscope, comprising: a sample stage on which a sample is placed; and a detection unit arranged in a direction in which the electron beam passes through the sample stage and detecting an image formed by interference of the electron beam. . 2. A scanning interference electron microscope according to claim 1, further comprising a magnifying lens between said sample stage and said detecting means for magnifying an image formed by interference. 3. The scanning interference electron microscope according to claim 1, wherein a biprism is used as said separating means.