JP3234441B2 - Atomic step observation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface observation technique for observing atomic steps on a crystal surface, and more particularly to a method for observing atomic steps on a silicon crystal (111) surface.

[0002]

2. Description of the Related Art Atomic steps are caused by a height difference of one atomic layer or several atomic layers, and are caused by a small angle between a low-index crystal plane and an actual crystal surface. Atomic steps can be said to be defects on a flat crystal surface, and affect the characteristics of a semiconductor element having a crystal surface or an interface as an active layer. Therefore, observing the atomic step distribution on the surface of a silicon crystal used as a substrate of a semiconductor element is
It is also important from a practical point of view. In addition, by observing the atomic steps, it is possible to obtain information on the behavior of atoms on the surface of the crystal and the interaction between the atomic steps. Therefore, the step observation is also important in terms of basic science.

Conventionally, as means for directly observing atomic steps on a crystal surface, a method of directly detecting step steps with a scanning tunneling microscope or an atomic force microscope, or a method such as a reflection electron microscope or a low-speed electron microscope is used. In addition, a method of detecting a change in contrast of an electron image caused by a step of an atomic step, a method of detecting a change in secondary electron intensity at a step end by a scanning electron microscope, and the like have been used. And these are all sensitive to atomic steps,
It can follow dynamic changes of atomic steps in vacuum.

[0004]

However, in the above-described atomic step observation method, when the actual crystal surface approaches the low index plane of the crystal, the step interval expands to several to several tens of μm, so that it is wide at low magnification. It is necessary to observe the area, and various problems occur. For example, in a scanning tunneling microscope or an atomic force microscope, detecting atomic steps with high sensitivity and scanning a crystal surface over a wide range are incompatible, and it becomes difficult to observe atomic steps. That is,
When the scanning range is as narrow as about 1 μm, it is difficult to find atomic steps at intervals of several tens of μm. In addition, in a reflection electron microscope, a low-speed electron microscope, and a scanning electron microscope, the width and contrast of the observed atomic step image are small. Therefore, when the observation range is widened, the image magnification decreases. Become.

As described above, the conventional method for observing atomic steps has a problem that it is difficult to observe atomic steps at intervals of several μm or more on the silicon crystal surface. Furthermore, with a reflection electron microscope or a low-speed electron microscope, once a sample is taken out to the atmosphere, the surface is contaminated or oxidized, so that electron diffraction does not occur and atomic steps can no longer be observed. There were also problems.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and has several μm on the surface of a silicon crystal (111).
Atomic steps distributed at intervals of m or more easily, and
It is an object of the present invention to provide an atomic step observation method capable of observing clearly.

[0007]

In order to achieve this object, in the present invention, a (111) silicon crystal is heated in a vacuum to a high temperature equal to or higher than a (7 × 7)-(1 × 1) phase transition temperature. Then, the silicon crystal is quenched to a temperature lower than the phase transition temperature by 100 ° C. or more, and (7 × 7) regions formed along the atomic steps on the silicon crystal surface and other (1 × 7) regions are formed. 1) Observe the region by distinguishing it from the secondary electron image.

In order to avoid contamination and oxidation of the crystal surface, germanium or silicon is vapor-deposited on the crystal surface quenched from the high temperature, and the crystal surface is covered with a thin amorphous film.

[0009]

In this atomic step observation method, when the (111) surface of a silicon crystal is heated in a vacuum, about 86
Above the (7 × 7)-(1 × 1) phase transition temperature of 0 ° C., the crystal surface has a (1 × 1) structure having the same period as the inside of the crystal, and below the phase transition temperature, a (7 × 7) period having a seven-fold period. Take advantage of the structure. When the surface having a (1 × 1) structure at a temperature higher than the phase transition temperature is set to a temperature lower than the phase transition temperature, (7
× 7) The structure grows with the atomic step ends as nuclei. At this time, the (7 × 7) structure can grow rapidly from the phase transition temperature to a temperature lower than 100 ° C. At a lower temperature, the growth rate becomes very slow. Therefore, by rapidly cooling to a temperature 100 ° C. or more lower than the phase transition temperature,
As shown in FIG. 1, the region where the (7 × 7) structure occurs can be limited to the vicinity of the atomic step. When the region of the (7 × 7) structure formed near this atomic step is observed separately from the region of the other (1 × 1) structure, (7 × 7)
× 7) By contouring the structure, the distribution image of the atomic steps can be easily and clearly observed. (7x
7) A secondary electron image observation using a scanning electron microscope can be used to distinguish between the structure and the (1 × 1) structure.

However, once the sample is taken out into the atmosphere, the above-mentioned (7 × 7) structure region and (1 × 1) structure region are broken by oxidation, and are distinguished by secondary electron images. You will not be able to attach it. Therefore, before the sample is taken out into the atmosphere, the sample surface is protected by covering it with an amorphous thin film having such a thickness that secondary electrons are not completely attenuated. On the surface of the silicon crystal under the amorphous film,
Since the regions of the (7 × 7) structure and the (1 × 1) structure are preserved, the regions of the underlying (7 × 7) structure and the (1 × 1) structure are distinguished by the secondary electron image through the amorphous film. Can be observed. In addition, since the amorphous film protects the silicon crystal surface from oxidation, the sample stored in the atmosphere can be observed many times.

[0011]

FIG. 2 shows the steps of an embodiment of the atomic step observation method according to the present invention. First, the observation sample was placed in a vacuum at (7 × 7)-(1 × 1) phase transition temperature (about 86
(0 ° C.) or higher. For the heating, it is preferable to use a method capable of stopping the heating instantly, for example, heating by direct energization to the sample, electron beam heating, laser beam heating, infrared heating, or the like. Next, the temperature of the observation sample is rapidly cooled to a temperature T2 lower than the phase transition temperature Tc by 100 ° C. or more. This is because the heating condition is changed from the set condition for T1 to T1.
This can be done by instantaneously switching to the setting condition for 2. However, (7 ×
7) When it is desired to greatly expand the structure area, the distance from T1 to T2
Alternatively, the temperature may be continuously controlled and changed. In this case, if the cooling rate is reduced to several degrees Celsius / second, the entire surface changes to a (7 × 7) structure, so care must be taken. Also, depending on how to set the temperature of T1 and T2,
The width of the (7 × 7) structure region can be changed. For example, if T1 is set to a high temperature of 1200 ° C. and T2 is set to a temperature realized after stopping the heating, the width of the (7 × 7) structure region can be minimized. On the other hand, T1 is just above the phase transition temperature Tc, and T2 is 10
If the temperature is set lower by 0 ° C., the width of the (7 × 7) structure region becomes wider. Next, the temperature of the observation sample is kept at room temperature or a temperature at which crystallization of the amorphous phase does not occur in vacuum, and an amorphous thin film is deposited. As the amorphous film, for example, silicon or germanium can be used. The thickness of the amorphous film is set to a thickness at which the secondary electrons generated from the silicon crystal surface are not greatly attenuated, preferably 2 nm or less. Observation sample obtained by the above steps, using a scanning electron microscope,
The (7 × 7) structure and the (1 × 1) structure are observed separately. Thereby, the distribution of atomic steps on the crystal surface can be observed.

In the above embodiment, an amorphous thin film is deposited on the surface of a silicon crystal after a (7 × 7) structure is formed along an atomic step. , Atomic step (7 × 7)
It is possible to observe using the structure. For example, if a heat treatment for forming a (7 × 7) structure as described above is performed in a sample chamber using an ultra-high vacuum scanning electron microscope, there is no need to expose the sample to the atmosphere, and the silicon crystal surface Observation of atomic steps is possible.

FIG. 3 is an image of an atomic step on the surface of the silicon crystal (111) observed according to the present embodiment. In this image, a dislocation appeared on the crystal surface was observed, and a spiral atomic step was observed. The observation conditions of this sample are as follows. First, 1 × 10
In an ultra-high vacuum of ~ ⁸ Pa, the sample was heated to 1260 ° C by direct application of direct current, and held at this temperature for 1 second.
The sample was quenched by stopping the energization. At this time, the cooling rate near the phase transition temperature was about 150 ° C./sec. Thereafter, a germanium thin film having a thickness of 1 nm was deposited in an ultra-high vacuum. For germanium, set the sample temperature to 20
If vapor deposition is performed at 0 ° C. or less, crystallization does not occur, and an amorphous film is obtained. Next, the sample surface was observed as a secondary electron image with a scanning electron microscope. In this secondary electron image,
The (7 × 7) structure area is brighter than the (1 × 1) structure area. Therefore, a (7 × 7) structural region formed along the atomic steps becomes a clear image, and atomic steps at 5 μm intervals are clearly observed. The width of the (7 × 7) structure region is 0.5 μm, and the atomic steps are easily identified even in a wide range observation (magnification: 2,000 times) of up to 50 μm.

[0014]

As described above, in the atomic step observation method according to the present invention, the silicon crystal (111)
Forming (7 × 7) structure regions along the atomic steps by heat treatment in a vacuum on the surface, and observing these regions and other (1 × 1) regions by secondary electron images Thereby, the atomic steps distributed on the silicon crystal (111) surface at intervals of several μm or more can be easily and clearly observed. As a result, a great deal of progress has been made in the research and development of semiconductor devices using silicon crystal (111) substrates and in basic research on the behavior of atomic steps.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a surface structure near an atomic step formed by an atomic step observation method according to the present invention.

FIG. 2 is a process chart in an embodiment of the atomic step observation method according to the present invention.

FIG. 3 shows a scanning electron microscope on a silicon crystal (111) surface to which an atomic step observation method according to the present invention has been applied.
It is a photograph observed .

Claims (3)

(57) [Claims] In the method for observing an atomic step on the surface of a silicon crystal (111), the silicon crystal is subjected to (7) in a vacuum.
X7)-heated to a high temperature equal to or higher than the (1 x 1) phase transition temperature, and then the silicon crystal is heated to 100 ° C above the phase transition temperature.
It is rapidly cooled to a temperature lower than or equal to ° C., and the (7 × 7) region formed along the atomic step on the silicon crystal surface and the other (1 × 1) region are observed separately by secondary electron images. An atomic step observation method, characterized in that: 2. The method according to claim 1, wherein said silicon crystal is placed in a vacuum at (7 × 7)-.
After heating to a high temperature of (1 × 1) phase transition temperature or higher, and then rapidly cooling the silicon crystal to a temperature of 100 ° C. or lower than the phase transition temperature, the surface of the silicon crystal was covered with a thin amorphous film. The atomic step observation method according to claim 1, wherein: 3. The atomic step observation method according to claim 2, wherein said amorphous thin film is formed by vapor deposition of germanium or silicon.