Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2000-05-15
2002-02-19
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S202000, C117S217000, C117S218000
Reexamination Certificate
active
06348095
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for pulling a single crystal and, more particularly, to a method for pulling a single crystal wherein a single crystal of silicon or the like is pulled by a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method) and an apparatus for pulling a single crystal used for the method for pulling a single crystal.
2. Description of the Relevant Art
At present, the majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like have been pulled by the CZ method.
FIG. 1
is a schematic sectional view of a conventional apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral
21
represents a crucible.
The crucible
21
comprises a bottomed cylindrical quartz crucible
21
a
and a bottomed cylindrical graphite crucible
21
b
fitted on the outer side of the quartz crucible
21
a.
The crucible
21
is supported with a support shaft
28
which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater
22
of a resistance heating type and a heat insulating mold
27
arranged around the heater
22
are concentrically arranged around the crucible
21
. The crucible
21
is charged with a melt
23
of a material for forming a crystal which is melted by the heater
22
. On the central axis of the crucible
21
, a pulling axis
24
made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal
35
is held by a holder
24
a.
These parts are arranged within a water cooled type chamber
29
wherein pressure can be controlled.
A method for pulling a single crystal
36
using the above-mentioned apparatus for pulling a single crystal is described below by reference to
FIGS. 1 and 2
. FIGS.
2
(
a
)-
2
(
d
) are partial enlarged front views schematically showing a seed crystal and the vicinity thereof in part of the steps in pulling a single crystal.
Although it is not shown in
FIG. 2
, after reducing the pressure in the chamber
29
, an inert gas is introduced into the chamber
29
so as to make an inert gas atmosphere under reduced pressure within the chamber
29
. Then, the material for forming a crystal is melted by the heater
22
and is maintained in the situation for a period of time so as to sufficiently release gas contained in the melt
23
.
While the pulling axis
24
is rotated on the same axis in the reverse direction of the support shaft
28
at a prescribed speed, the seed crystal
35
held by the holder
24
a
is caused to descend and is brought into contact with the melt
23
so as to make the front portion of the seed crystal
35
adhere smoothly to the melt
23
. Then, the pulling of the single crystal
36
from the melt
23
is started (the seeding step) (FIG.
2
(
a
)).
In making a crystal grow at the front portion of the seed crystal
35
, the pulling axis
24
is pulled at a higher speed than the below-described formation speed of a main body
36
c.
The crystal is narrowed to have a prescribed diameter, leading to the formation of a neck
36
a
(the neck formation step) (FIG.
2
(
b
)).
By slowing down the pulling speed of the pulling axis
24
(hereinafter, simply referred to as the pulling speed), the neck
36
a
is made to grow to have a prescribed diameter, leading to the formation of a shoulder
36
b
(the shoulder formation step) (FIG.
2
(
c
)).
By lifting the pulling axis
24
at a fixed rate, the main body
36
c
having a uniform diameter and a prescribed length is formed (the main body formation step) (FIG.
2
(
d
)).
Although it is not shown in
FIG. 2
, in order to prevent induction of high-density dislocations to the single crystal
36
by a sudden change in temperature at the end, the diameter of the single crystal
36
is gradually decreased so that the temperature of the whole single crystal
36
is gradually lowered, leading to the formation of an end-cone. Then, the single crystal
36
is separated from the melt
23
. After finishing the above steps, cooling the single crystal
36
is the end of the pulling process of the single crystal
36
.
One of the important steps in the pulling of the single crystal
36
is the above-mentioned neck formation step. The object of the neck formation step is described below.
Before carrying out the seeing step, the front portion
35
a
of the seed crystal
35
is preheated to some extent. Ordinarily, there is a difference of 100° C. or more between the preheating temperature (about 1300° C. and less) and the melting point of the seed crystal
35
(about 1410° C.). Therefore, in dipping the seed crystal
35
into the melt
23
, the seed crystal
35
experiences a steep rise in temperature, leading to the induction of dislocations generated by a thermal stress to the front portion
35
a
thereof. It is necessary to make the single crystal
36
grow after excluding the dislocations which inhibit single crystal growth. Since the dislocations generally tend to grow in the vertical direction to the growth interface of the single crystal
36
, the shape of the growth interface (the front plane of the neck
36
a
) is made downward convex through the neck formation step, so as to exclude the dislocations.
In the neck formation step, the faster the pulling speed is made, the smaller the diameter of the neck
36
a
can be made. By making the shape of the growth interface more downward convex, the propagation of the dislocations can be prevented and the dislocations can be efficiently excluded.
In the above conventional method for pulling a single crystal, the seed crystal
35
having a diameter of 12 mm or more has been generally used in order to pull the single crystal
36
having a diameter of about 6 inches and a weight of 80 kg or so. In this case, the larger the diameter of the neck
36
a
is, the more safely the single crystal
36
can be supported, while the smaller the diameter of the neck
36
a
is, the more efficiently the dislocations can be excluded. In order to meet both of the requirements which are contrary to each other, the neck
36
a
having a diameter of 3 mm or so has been used so far. Recently, however, in order to produce a more highly integrated semiconductor device at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal
36
having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. In this case, the neck
36
a
having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal
36
and breaks, resulting in the falling of the single crystal
36
.
In growing the above heavy single crystal
36
, the diameter of the neck
36
a
needs to be about 6 mm or more for safety, which is calculated from the silicon strength (about 16 kgf/mm
2
) in order to prevent the occurrence of troubles such as a fall of the single crystal
36
. However, when the diameter of the neck
36
a
is 6 mm or more, the dislocations which are induced in dipping the seed crystal
35
into the melt
23
cannot be sufficiently excluded.
In order to solve the above problem, a method for growing a heavy single crystal was proposed in Japanese Kokai No. 62-288191, wherein the diameter is once increased after growing the neck
36
a,
and is reduced and is increased again, so as to form a high-strength holding portion, which is mechanically held. It is possible to hold the heavy single crystal by the method, but a special jig, control, and the like for mechanical holding exclusive use are required in order to perform the mechanical holding in the method. In addition, when the mechanical holding is conducted on the high-strength holding portion, there is a high possibility that shaking or the like generates dislocations in the growing portion, resulting in a lower yield of the products.
The applicant of the present invention previously proposed a method
Morita Hiroshi
Watanabe Hideki
Hiteshew Felisa
Sumitomo Metal Industries Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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