Single crystal pull-up apparatus

Details Single crystal pull-up apparatus Single crystal pull-up apparatus

2000-07-07

2002-01-15

Hiteshew, Felisa (Department: 1765)

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

Apparatus

For crystallization from liquid or supercritical state

C117S200000, C117S208000, C117S217000, C117S222000, C117S900000

Reexamination Certificate

active

06338757

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an apparatus for pulling a single crystal and, more particularly, to an apparatus for pulling a single crystal with which a silicon single crystal used as a semiconductor material is pulled in its low defect density state.
BACKGROUND ART
There are various methods for growing a single crystal, and one of them is a single crystal growth method called the Czochralski method (hereinafter, referred to as the CZ method).
FIG. 1
is a diagrammatic sectional view of an apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral
1
represents a crucible.
The crucible
1
comprises a bottomed cylindrical quartz crucible
1
a
and a bottomed cylindrical graphite crucible
1
b
fitted on the outer side of the quartz crucible
1
a
. The crucible
1
is supported with a support shaft
8
which rotates in the direction shown by the arrow in the figure at a prescribed speed. A heater
2
of a resistance heating type and a heat insulating mold
7
arranged around the heater
2
are concentrically arranged around the crucible
1
. The crucible
1
is charged with a melt
3
of a material for forming a crystal which is melted by the heater
2
. On the central axis of the crucible
1
, a pulling axis
4
made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal
5
is held by a holder
4
a
. These parts are arranged within a water cooled type chamber
9
wherein pressure can be controlled.
A method for pulling a single crystal
6
using the above-described apparatus for pulling a single crystal is described below. By reducing the pressure in the chamber
9
and introducing an inert gas thereto, the atmosphere in the chamber
9
is made to be an inert gas atmosphere under reduced pressure. Then, the material for forming a crystal is melted by the heater
2
and is left standing for a period of time so as to release gas in the melt
3
sufficiently.
While the pulling axis
4
is rotated on the same axis in the reverse direction of the support shaft
8
at a prescribed speed, the seed crystal
5
held by the holder
4
a
is caused to descend and is brought into contact with the melt
3
so as to make the seed crystal
5
partially melt into the melt
3
. Then, the single crystal
6
is grown at the lower end of the seed crystal
5
.
In growing the single crystal
6
, the seed-narrowing (
6
a
) is conducted in order to make the single crystal
6
dislocation-free. Then, a shoulder portion
6
b
is grown so as to obtain the single crystal
6
having a required diameter in a body portion
6
c
. When the diameter of the single crystal
6
becomes a required one, the shoulder formation is finished. While the diameter is kept uniform, the body portion
6
c
is grown. After the body portion
6
c
is grown to a prescribed length, the tail-narrowing is conducted in order to separate the single crystal
6
from the melt
3
in the dislocation-free state. Then, the single crystal
6
separated from the melt
3
is cooled under prescribed conditions. Wafers manufactured by processing the single crystal
6
thus obtained are used as a substrate material of various semiconductor devices.
In the silicon single crystal pulled through the above steps, defects called infrared scatterers (COP and FPD), dislocation clusters or the like sometimes exist. These defects are not newly formed within the crystal by the later heat treatment. They are already formed during crystal pulling, which are also called grown-in defects.
FIG. 2
is a diagram showing the general relation between the pulling speeds during single crystal growth and occurrence positions of crystal defects. As is shown in
FIG. 2
, infrared scatterers
21
among the grown-in defects observed in the evaluation after crystal growth are detected inside a ring region of oxidation-induced stacking fault (OSF)
22
, a kind of thermally-induced defect. Defects called dislocation clusters
24
among the grown-in defects are detected outside the ring region
22
. And a defect-free region
23
exists close to the outside of the ring region (R-OSF)
22
. The occurrence region of the ring region (R-OSF)
22
depends on the pulling speed during single crystal growth. As the pulling speed is made lower, the region wherein the ring region (R-OSF)
22
appears shrinks inward from the outer side of the crystal.
The above OSF is an interstitial dislocation loop which occurs during oxidative heat treatment. When the OSF is generated and grows on a wafer surface which is an active region of a device, it causes a leakage current, so that it becomes a defect which deteriorates device properties. Therefore, hitherto, the high-density region of OSF is push out toward the perimeter of the crystal by controlling the position of the R-OSF so as to move toward the perimeter thereof during single crystal growth.
However, recently, the adverse effects of the OSF on devices are controlled since the device manufacturing process is conducted at lower temperatures and a crystal contains less oxygen. Therefore, the OSF is inconsiderable as a factor which deteriorates the device properties. On the other hand, the infrared scatterers among the grown-in defects are a factor which deteriorates the time zero dielectric breakdown, and the dislocation clusters are a factor which remarkably deteriorates the device properties. Recently it is an important problem to reduce the density of those grown-in defects within a crystal.
Accordingly, it is attempted to obtain high-quality devices by utilizing the region wherein almost no defects to deteriorate the device properties are detected, or, the defect-free region existing close to the inside or outside of the ring region (R-OSF)
22
. But since the above defect-free region is a very limited region, it is difficult to utilize it effectively. In order to deal with these problems, some methods were proposed.
For example, in Japanese Patent Laid-Open No. 08-330316, it is disclosed that a crystal wherein only the outside region of a ring region (R-OSF)
22
spreads all over the surface thereof can be grown by improving the crystal growth conditions so as to generate no dislocation clusters. However, there is a possibility to achieve this only with a very limited crystal growth condition, or, only by controlling the pulling speed within a very limited range to a certain temperature gradient. It is an extremely severe condition in the silicon single crystal growth wherein a crystal will have a larger and larger diameter and mass production thereof is required.
In Japanese Patent Laid-Open No. 07-257991, and Journal of Crystal Growth, 151 (1995) pp.273-277, it is disclosed that the outside region of a R-OSF can be generated by making the temperature gradient in the pulling axis direction large and pulling a single crystal at a high speed so as to annihilate the R-OSF on the inside of the crystal. However, reduction of grown-in defects within a crystal plane is not considered at all. Even if the R-OSF is caused to shrink inward, dislocation clusters exist in the outside region of the R-OSF as before. Since the dislocation clusters greatly deteriorate the device properties, it cannot be said that high-quality wafers are provided.
DISCLOSURE OF INVENTION
The present invention was developed in order to solve the above problems, and it is an object to provide an apparatus for pulling a single crystal with which a single crystal having a low density of grown-in defects called infrared scatterers, dislocation clusters or the like can be grown.
The present inventors examined the occurrence situation of dislocation clusters to the R-OSF occurrence position within a single crystal grown with conventional conditions or a single crystal wafer and the width thereof. In order to make clear the R-OSF occurrence position within a wafer surface, the distance from the center of the crystal (wafer) to the perimeter (or, the crystal radius) is represented by R, while the R-OSF occurrence position in the radial direction within the crystal is represented by r. For

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