Methods for pulling a single crystal

Details Methods for pulling a single crystal Methods for pulling a single crystal

1998-12-22

2001-04-03

Utech, Benjamin L. (Department: 1765)

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

Processes of growth from liquid or supercritical state

Having pulling during growth

C117S217000, C117S902000

Reexamination Certificate

active

06210477

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for pulling a single crystal and, more particularly, to methods 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.
2. Description of the Relevant Art
At present, the majority of silicon single crystals (ingots) 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 sectional view of a conventional apparatus for pulling a single crystal using the CZ method, and in the figure, reference numeral
11
represents a crucible.
The crucible
11
comprises a bottomed cylindrical quartz crucible
11
a
and a bottomed cylindrical graphite crucible
11
b
fitted on the outer side of the quartz crucible
11
a
. The crucible
11
is supported with a support shaft
18
which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater
12
of a resistance heating type and a heat insulating mold
17
, arranged around the heater
12
, are concentrically arranged around the crucible
11
. The crucible
11
is charged with a melt
13
of a material for forming a crystal which is melted by the heater
12
. On the central axis of the crucible
11
, a pulling axis
14
made of a pulling rod or wire, which is suspended, and at the front of the pulling rod or wire, a seed crystal
15
is held by a holder
14
a
. These parts are arranged within a water cooled type chamber
19
wherein pressure of the chamber can be controlled.
A method for pulling a single crystal
16
using the above-mentioned apparatus for pulling a single crystal is described below by reference to
FIGS. 1 and 2
. FIGS.
2
(
a
)-(
d
) are partially enlarged front views diagrammatically showing the seed crystal
15
and the steps in a conventional method for pulling a single crystal.
Although it is not shown in
FIG. 1
, an electric current is applied to the heater
12
so as to melt the material for forming a crystal. after reducing the pressure in the chamber
19
. Then, an inert gas is introduced into the chamber
19
so as to make an inert gas atmosphere at a prescribed pressure within the chamber
19
.
While the pulling axis
14
is rotated on the same axis in the reverse direction of the support shaft
18
at a prescribed speed, the seed crystal
15
, held by the holder
14
a
, is descended and is brought into contact with the melt
13
so as to make the front portion
15
a
of the seed crystal
15
partially melt into the melt
13
. Then, the pulling of the single crystal
16
from the melt
13
is started. This is referred to as the seeding as shown in (FIG.
2
(
a
).
In making a crystal grow at the front portion
15
a
of the seed crystal
15
, the pulling axis
14
is pulled at a higher speed than the below-described pulling speed in the formation of a main body
16
c
. The crystal is narrowed to have a prescribed diameter, leading to the formation of a neck
16
a
. This is referred to as the necking step (FIG.
2
(
b
)).
By slowing down the pulling speed of the pulling axis
14
(hereinafter, simply referred to as the pulling speed), the neck
16
a
is made to grow to have a prescribed diameter, leading to the formation of a shoulder
16
b
(FIG.
2
(
c
)).
By pulling the pulling axis
14
at a fixed rate, the main body
16
c
having a uniform diameter and a prescribed length is formed (FIG.
2
(
d
)).
Although it is not shown in
FIG. 2
, in order to prevent induction of high density dislocation to the single crystal
16
by a sudden temperature change when the separation of the single crystal
16
from the melt
13
approaches, the diameter of the single crystal
16
is gradually decreased so that the temperature of the whole single crystal
16
is gradually lowered, leading to the formation of an end-cone. Next, the single crystal
16
is separated from the melt
13
. Finally, the single crystal
16
is cooled at the end of the pulling of the single crystal
16
.
One of the important steps in the pulling of the single crystal
16
is the above-mentioned necking step called the Dash method (J. Appl. Phys. 30 [4] (1959) W. C. Dash. p.459-473) (FIG.
2
(
b
)).
The object of the necking step is described below. In the above seeding step (FIG.
2
(
a
)), the front portion
15
a
of the seed crystal
15
is preheated to some extent and is brought into contact with the melt
13
. 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
15
(about 1410° C.). Therefore, in contact with the melt
13
, the front portion
15
a
of the seed crystal
15
has a steep temperature gradient, leading to the induction of dislocation caused by a thermal stress thereto. It is necessary to make the single crystal
16
grow after excluding the dislocation which propagates and inhibits single crystal growth. Since the dislocation generally tends to grow in the vertical direction to the growth interface of the single crystal
16
, the shape of the growth interface (the front plane of the neck
16
a
) is made downward convex, so as to exclude the dislocation outward.
In the pulling of a single crystal, the faster the pulling speed, the smaller the diameter of the single crystal, or the more downwardly convex the shape of the growth interface of the single crystal. Therefore, in the above necking step, it is desired that the pulling speed be made as fast as possible to make the diameter of the neck
16
a
smaller, or to make the shape of the growth interface more downwardly convex, so as to efficiently exclude the dislocation outward.
In the above conventional method for pulling a single crystal, the seed crystal
15
having a diameter of, for example about 12 mm has been generally used in order to pull the single crystal
16
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
16
a
is, the more safely the single crystal
16
can be supported, while the smaller the diameter of the neck
16
a
is, the more efficiently the dislocation can be excluded. In order to meet both of the requirements, the neck
16
a
having a diameter of 3 mm or so is selected.
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
16
having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. When the diameter of the single crystal
16
is made larger, the weight of the shoulder
16
b
band tail inevitably becomes heavier. It becomes necessary to lengthen the main body
16
c
which can form a product in order to obtain the profitable yield. In other words, it becomes necessary to grow a heavy single crystal.
When the requirement is satisfied, the neck
16
a
having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal
16
and breaks, resulting in the falling of the single crystal
16
.
In growing the above heavy single crystal
16
, the diameter of the neck
16
a
needs to be about 5 mm or more in order to prevent the occurrence of troubles such as a fall of the single crystal
16
and to pull the single crystal
16
safely, which is calculated from the silicon strength (about 16 kgf/mm
2
). However, when the diameter of the neck
16
a
is 5 mm or more, the dislocation which is induced in contact of the seed crystal
15
with the melt
13
cannot be sufficiently excluded outward.
In order to solve the 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
16
a
, and is reduced and is increased again, so as to form a

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