JPH0920587A - Molecular beam source - Google Patents

Abstract

(57) [Abstract] [Purpose] The purpose of the molecular beam source is to stabilize the molecular beam intensity. [Composition] A crucible for accommodating a source material and a shielding plate inserted into the crucible so as to cover the source material are provided, and a molecular beam generated from the source material is emitted to the outside through the shielding plate. In this case, the shielding plate is configured to be movable in the crucible when the source material is deformed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molecular beam source used in a molecular beam crystal growth apparatus. A molecular beam crystal growth apparatus (hereinafter referred to as an MBE apparatus) is widely used because it can accurately and easily control the composition ratio and film thickness of a semiconductor thin film crystal. In order to satisfy the above requirement, it is necessary to further improve the controllability.

[0002]

2. Description of the Related Art FIG. 4 is a sectional view showing the main part of an MBE device. The semiconductor substrate 7 and the molecular beam source 8 are placed in the vacuum container 6.
Are arranged so as to face each other, and the semiconductor substrate 7 is irradiated with the molecular beam emitted from the molecular beam source 8 to grow a thin film crystal. A plurality of molecular beam sources 8 are provided in accordance with the types of elements that make up the thin film crystal. By adjusting the intensity of the molecular beam emitted from each molecular beam source, the film thickness of the thin film crystal, the composition ratio of the constituent elements, etc. Are trying to control. For example, in the case of growing a GaAlAs crystal, three molecular beam sources each containing a source material composed of Ga, Al and As are used.

FIG. 5 is a cross-sectional view of a main part of a conventional molecular beam source, showing a case where As is used as a source material. In the figure, 1 is a crucible, 2 is a heater arranged outside the crucible 1, and 3 is an As source material. Since As is a sublimable substance and is solid at room temperature, it is stored in the crucible 1 as a source material formed according to the internal shape of the crucible 1.

When the molecular beam source having the above-mentioned structure is set in the MBE apparatus and the crucible 1 is heated by the heater 2, the As source material 3 is vaporized in a solid state to be a molecular beam and injected to the outside. The temperature of the crucible 1 is adjusted to control the evaporation amount of the As source material 3, and thereby the molecular beam intensity is controlled.

[0005]

In FIG. 5, (a) is
The state before evaporation of the As source material 3, (b) and (c) schematically show the state when evaporation proceeds, and the As source material 3 shrinks and deforms as the evaporation progresses. I understand that it will go. The cause of the shape change of the As source material 3 as shown in the figure is considered as follows.

As described above, As has a sublimation property, so that evaporation proceeds from the surface in a solid state. The evaporation amount is proportional to the surface area of the As source material 3, but also depends on the surface temperature. On the other hand, the As source material 3 is arranged in the crucible 1 when it is housed in the crucible 1.
The surface temperature of the source material 3 is not uniform depending on the distance from the inner wall of the crucible and the like. Therefore, evaporation locally progresses in the portion where the temperature rises, the consumption amount of As in this portion increases, and the shape of the As source material 3 changes. As the evaporation progresses, the process described above progresses, as shown in FIG.
It is considered that the shape change of the As source material 3 as shown in (c) occurs.

FIG. 6 shows the time dependence of As molecular beam intensity, where the vertical axis represents the As molecular beam intensity and the horizontal axis represents the time lapse after the start of use of the As molecular beam source in week units. Shows.
The As molecular beam intensity shows irregular changes, and when the evaporation of As source material 3 proceeds, it seems that the surface area of the As source material 3 has decreased, as shown in Figs. 5 (b) and 5 (c). Nevertheless, the molecular beam intensity increases moderately. This is because the evaporation of As occurs not only from the surface of the As source material 3 but also from inside the As source material 3, and as a result, the vicinity of the surface of the As source material 3 changes to a porous structure, which is contrary to the apparent reduction in surface area. This is probably due to the increase in the specific surface area. Further, since the portion having the porous structure near the surface is brittle, the shape may collapse and the surface area may suddenly change due to a slight impact. This is due to the irregular molecular beam intensity shown in FIG. It is thought to be the cause of change.

As described above, in the conventional molecular beam source, when the sublimable substance is used as the source material, the intensity of the molecular beam is not constant, and as a result, the amount of As deposited on the semiconductor substrate becomes unstable. As a result, there arises a problem that the film thickness and composition ratio of the thin film crystal cannot be controlled accurately.

Therefore, the present invention aims to stabilize the molecular beam intensity.

[0010]

Means for Solving the Problems To solve the above problems, a molecular beam generated from a source material is provided, which comprises a crucible for accommodating a source material and a shielding plate inserted into the crucible so as to cover the source material. Is a molecular beam source adapted to be emitted to the outside through a shield plate, wherein the shield plate is movable in the crucible according to the deformation of the source material. , Or in the above molecular beam source,
The shielding plate has at least one hole and is brought into contact with the upper surface of the source material by its own weight in the crucible.

[0011]

[Function] Since the shielding plate is configured to be movable while being in contact with the source material, when the source material is reduced and deformed by evaporation, the shielding plate moves downward accordingly and is always in contact with the source material. As a result, the source material is confined in the crucible. In order to make the shield plate movable in contact with the source material, for example, if the diameter of the shield plate is set to a value smaller than the inner diameter of the crucible and the shield plate is placed on the source material by its own weight. Good. With such a configuration, the shielding plate functions as a heat conduction plate, and the temperature of the surface of the source material becomes more uniform. Furthermore, the shielding plate prevents the source material from being deformed by constantly applying pressure to the source material from above, which prevents irregular changes in the surface area of the source material when evaporation of the source material progresses. It will be suppressed. The vaporized source material is injected through a gap between the shield plate and the crucible or a hole provided in the shield plate. As described above, the irregular fluctuation of the evaporation amount of the source material is suppressed as compared with the conventional case, and the strength of the molecular beam emitted to the outside is stabilized.

[0012]

EXAMPLE FIG. 1 is a cross-sectional view of an essential part of a molecular beam source showing an example of the present invention, which uses an As source as a source material.
Figure 1 (a) shows the state of the molecular beam source before evaporation of the As source. In the figure, the crucible 1 has a cylindrical shape with an inner diameter of 80 mm and a length of 140 mm and is made of BN. C in addition to BN, or
A material having excellent heat resistance such as AlN may be used. The As source material 3 is formed into a cylindrical shape according to the internal shape of the crucible 1 and is housed in the crucible 1. The volume of As source material 3 is 600cc. The shield plate 4 has a disk shape with a diameter of 760 mm and a thickness of 15 mm and is made of tantalum (Ta) having a weight of 800 g. FIG. 2 is a plan view of the shield plate. A large number of holes 5 having a width of 2 mm are concentrically provided in order to eject As vaporized in the crucible 1. The shape and position of the holes and the number thereof are not limited to this example.

As shown in FIG. 1A, the shielding plate 4 is placed on the As source material 3. Since the diameter of the shielding plate 3 is slightly smaller than the inner diameter of the crucible 1, the shielding plate 3 has a function of a dropping pig, and the As source material 3 is brought into contact with the upper surface of the As source material 3 by its own weight so that the As source material 3 is inside the crucible 1. The As source material 3 is always confined and pressure is applied from above. In this state, the crucible 1 by the heater 2
When heated, the temperature of the As source material 3 rises and evaporates in the solid state and is injected to the outside through the holes 5 provided in the shielding plate 4. As source material 3 gradually shrinks as the consumption of As increases due to evaporation, as shown in Figs. 1 (b) and (c), it is clear from comparison with Figs. 5 (b) and (c). Thus, irregular changes in shape are suppressed. This is As source material 3
Is confined in the crucible 1 while being always in contact with the shield plate 4, the heat of the inner wall of the crucible 1 is directly transferred to the As source material 3 through the shield plate 4, and the surface temperature is made more uniform. As a result, the local change in the amount of evaporation on the surface of the As source material 3 is suppressed, and further, since the shielding plate 4 constantly applies pressure to the upper surface of the As source material 3 by its own weight, it becomes brittle due to evaporation. This is because the surface shape of the As source material 3 is shaped so as to be crushed and irregular shape changes are suppressed.

FIG. 3 shows the time dependence of the As molecular beam intensity when the above molecular beam source is used. The horizontal axis indicates the time from the start of use of the molecular beam source in units of weeks, as in FIG. The amount of change in As molecular beam intensity is within the range of about 1%, and neither the irregular change nor the gradual increase phenomenon as shown in FIG. 6 is seen. This is due to the shielding plate 4 as described above.
This is because the surface temperature of the As source material 3 is uniform, the change in shape is suppressed, and the surface area is constant.

In the above embodiment, the shield plate is provided with holes, but the hole may not be provided if a sufficient amount of source evaporation can be obtained from the gap between the shield plate and the crucible. Further, although the shielding plate is made of Ta in the above embodiment, the present invention is not limited to this.
A refractory metal having excellent heat resistance such as Mo or W can be used. Regarding the weight of the shielding plate, the volume and weight of the source material,
Different values can be used depending on the inner volume of the crucible and the like. When the pressure applied to the source material is further increased and the weight of the shielding plate is not sufficient, a weight may be placed on the shielding plate. Further, in the above-mentioned examples, the case where As is used as the source material is described, but when a sublimable substance other than As, for example, Cr or the like is used as the source material, by using a shielding plate having a similar structure, The line strength can be stabilized.

[0016]

As described above, according to the present invention, it is possible to suppress irregular changes in the temperature distribution and shape of the source material, so that a stable molecular beam intensity can be obtained and the quality of the semiconductor thin film crystal can be improved. It is useful for improving

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a main part of a molecular beam source according to an embodiment of the present invention, in which (a) is a view showing a state before As source evaporation, (b),
(c) is a diagram showing the state of evaporation of As source

FIG. 2 is a plan view of the shield plate.

FIG. 3 is a diagram showing the time dependence of As molecular beam intensity in this example.

FIG. 4 is a sectional view of an essential part of the MBE device.

FIG. 5 is a cross-sectional view of a main part of a molecular beam source in a conventional example, (a) is a diagram showing a state before As source evaporation, (b), (c)
Is a diagram showing the state of evaporation of As source

FIG. 6 is a diagram showing the time dependence of As molecular beam intensity in a conventional example.

[Explanation of symbols]

 1 Crucible 3 As Source Material 2 Heater 4 Shielding Plate 5 Hole 7 Semiconductor Substrate 6 Vacuum Container 8 Molecular Beam Source

Claims (2)

[Claims] 1. A crucible for containing a source material, and a shield plate inserted into the crucible so as to cover the source material,
A molecular beam source configured to emit a molecular beam generated from a source material to the outside through a shield plate, the shield plate being movable in the crucible according to the deformation of the source material. A molecular beam source characterized in that 2. The shield plate has at least one hole,
The molecular beam source according to claim 1, wherein the molecular beam source is in contact with the upper surface of the source material by its own weight in the crucible.