Molecular beam source and molecular beam epitaxy apparatus

Details Molecular beam source and molecular beam epitaxy apparatus Molecular beam source and molecular beam epitaxy apparatus

2001-02-02

2002-11-05

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Apparatus

C117S108000, C117S900000

Reexamination Certificate

active

06475278

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2000-025445 filed on Feb. 2, 2000, whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a molecular beam source and a molecular beam epitaxy apparatus, more particularly, to a molecular beam source for accommodating a molecular beam generating material therein and thermally evaporating or subliming the material to generate a molecular beam in a molecular beam epitaxy (referred to as MBE hereinafter) technique and a molecular beam epitaxy apparatus using the molecular beam source.
2. Description of Related Art
The MBE technique is a technique for generating a molecular beam by evaporating or subliming a high purity material and growing crystals on a GaAs substrate or the like in a high vacuum. It is generally used for forming semiconductor thin films of compound semiconductor devices such as semiconductor lasers and is now under research and development for further improvement.
In the MBE technique, to reduce impurities remaining in a vacuum chamber is important in production of semiconductor thin films. For this purpose, exhausters have been improved and chamber baking has been implemented in order to obtain good semiconductor thin films.
However, substances adhering to sites other than a substrate such as a shroud (a cryo panel) and the like during discharge of gasified materials and/or during crystal growth come off and fall in a molecular beam source (also referred to as “molecular beam source cell” or simply “cell”) when liquid nitrogen is removed from the shroud. The fallen substances are re-evaporated at the next growth of crystals and result in increase of residual impurities in the vacuum chamber and a possible decline in the quality of semiconductor thin films. The re-evaporated substances may also enter a heater for heating the material mounted to a crucible of the molecular beam source cell and a lead line of a thermocouple for measuring temperature and cause troubles such as insulation failure.
To cope with this, measures have been taken such as inclining the vacuum chamber for preventing substances adhering to the shroud around the substrate and the like from falling into the cell even if they come off. Typically, the cell has a crucible in which a molecular beam generating material is fed and a heater disposed to surround the crucible almost entirely for evaporating the molecular beam generating material fed in the crucible.
With this construction, however, cells attached to upper ports of the vacuum chamber are inclined more.
Accordingly, if crucibles
901
and
902
of conventional structures shown in FIG.
11
and
FIG. 12
are used, the crucibles
901
and
902
can only accommodate a decreased amount of the molecular beam generating material. As a result, the molecular beam generating material is required to be fed an increased number of times, which results in an increase in the number of maintenance operations, a decline in the availability of the MBE apparatus and an increase in production costs.
If the chamber is further inclined and the cell is mounted to the port at an angle such that an inlet opening of the cell faces toward a direction lower than a horizontal line, the crucibles
901
and
902
of the conventional structures shown in
FIG. 11 and 12
cannot be used for a melt-type molecular beam generating material but can be used only for a sublime-type solid molecular beam generating material.
Published Japanese Translation of PCT International Publication for Patent Application No. HEI 11(1999)-504613 discloses a unibody crucible
903
having a negative draft orifice
904
as shown in FIG.
13
. With this construction, even if the cell is horizontally laid, the melt-type molecular beam generating material can be used.
Another conventional structure for a crucible is shown in FIG.
14
. The crucible
910
is comprised of a molecular beam generating material accommodating section
913
, a molecular beam shape defining section
912
, and a bent portion
908
formed therebetween, as shown in FIG.
14
. The crucible
910
has a structure such that the molecular beam generating material accommodated in the molecular beam generating material accommodating section
913
does not face an opening
905
directly, that is, the molecular beam generating material accommodated in the molecular beam generating material accommodating section
913
cannot be seen from the opening
905
.
In the above mentioned crucibles
901
and
902
, if the chamber is inclined for preventing substances adhering to the shroud or the like from falling into the crucible, the amount of the molecular beam generating material that can be fed in the crucible (i.e., the capacity of the crucible) becomes smaller in a cell attached to an upper port. The number of maintenance operations for feeding the material increases, the machine operating time decreases and the production costs increase.
Also, there is a problem in that the melt-type molecular beam generating material cannot be used at a port which causes the opening of the cell to face in a downward direction.
Further, if the cell is set substantially horizontally using the crucible
903
of the conventional structure shown in
FIG. 13
, an evaporation area of the molecular beam generating material changes and the intensity of the molecular beam changes as the molecular beam generating material is consumed and the liquid level of the material drops. Usually, the molecular beam intensity is measured at regular intervals and compensated by adjusting the temperature of the heater. With a crucible having a structure such that the evaporation area of the molecular beam generating material is liable to change, the measurement and compensation of the molecular beam intensity must be carried out more often, which results in a decrease in the availability of the apparatus and an increase in the production costs.
Further, if the crucible
910
having the structure shown in
FIG. 14
is used and the molecular beam shape defining section
912
is positioned substantially horizontally, the molecular beam intensity does not change owing to a drop in the liquid level of the molecular beam generating material, but, in order to increase the feed amount of the molecular beam generating material, the bent portion
908
of the crucible
910
need to be located at a position farther from a substrate than a cell port flange of the vacuum chamber so as to avoid contact with an outer surface of the vacuum chamber. Accordingly, if the opening
905
of the crucible
910
is set at the same position as an opening of a conventional cell, the distance from a bent portion
908
of the crucible
910
to the opening
905
becomes longer and a thinner molecular beam is emitted from the opening
905
of the crucible
910
. For this reason, the crucible
910
of the above-described structure is suitable for laboratory-scale MBE apparatuses for small substrates, while it cannot provide a uniform film thickness in industrial-scale MBE apparatus for large substrates.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, an object of the present invention is to provide a molecular beam source which is capable of accommodating a large amount of the molecular beam generating material and also provides a uniform film thickness on a large substrate and a molecular beam epitaxy apparatus using the molecular beam source.
The present invention provides a molecular beam source comprising a crucible having an opening, and a heater mounted to the crucible for evaporating by heating a molecular beam generating material accommodated in the crucible to emit a molecular beam from the opening, wherein the crucible has an accommodating section for accommodating the molecular beam generating material; a bent portion provided between the opening and the accommodating section so that the molecular beam generating material accom

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