High-quality silicon single crystal and method of producing...

Details High-quality silicon single crystal and method of producing... High-quality silicon single crystal and method of producing...

2000-02-24

2003-02-04

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

Reexamination Certificate

active

06514335

ABSTRACT:

TECHNICAL FIELD
This invention relates to silicon single crystals for use as semiconducting materials. More particularly, it is directed to a high-quality silicon single crystal that is grown by a Czochralski method (hereinafter referred to as “CZ method”) and that is excellent in device characteristics, and to a method of producing such a high-quality silicon single crystal.
BACKGROUND ART
A variety of methods are available to grow silicon single crystals for use as semiconducting materials. Among these methods, the CZ method is extensively used.
FIG. 1
is a schematic sectional view of a single crystal producing apparatus used for producing single crystals by a normal CZ method. As shown in
FIG. 1
, a crucible
1
comprises a quartz-made, bottomed, cylindrical inner layer holding container
1
a
, and a graphite-made, similarly bottomed and cylindrical outer layer holding container
1
b
that is fitted over the outside of the inner layer holding container
1
a
. The constructed crucible
1
is supported by a support shaft
1
c
that is rotated at a predetermined speed. Outside the crucible
1
is set a heater
2
, which is provided in the form of a concentric cylinder. The crucible
1
is charged with a melt
3
that is a raw molten material heated by the heater
2
. A pulling shaft
4
, such as a pull rod or a wire, is provided at the center of the crucible
1
. A seed chuck and a seed crystal
5
are attached to the distal end of the pulling shaft
4
, and the seed crystal
5
is brought into contact with the surface of the melt
3
in order to grow a single crystal
6
. Further, by pulling the seed crystal
5
at a predetermined rate using the pulling shaft
4
while rotating the pulling shaft
4
in a direction opposite to the direction of the crucible
1
rotated by the support shaft
1
c
, the melt
3
is solidified at the distal end of the seed crystal
5
, thereby gradually growing the single crystal
6
.
For single crystal growth, a seed-constricting step is carried out first so as to make the crystal dislocation-free. Thereafter, to secure a body diameter of the single crystal, a shoulder is formed, and when the body diameter has been obtained, a shoulder-changing step is performed. Then, the single crystal growing process is shifted to the single crystal body-growing step while maintaining the obtained body diameter. When the single crystal has been grown to a predetermined length while maintaining the body diameter, a tail constricting step is carried out so as to separate the single crystal from the melt in the dislocation-free state. Thereafter, the single crystal separated from the melt is taken out of the puller, and cooled under a predetermined condition, and processed into wafers. The wafers thus processed from the single crystal are used as substrate materials for the preparation of various devices.
In an in-plane area of a wafer that is processed through the above-described steps, there may occur, in some cases, oxidation-induced stacking faults (hereinafter referred to as “OSF”) as defects appearing through heat treatments. Ring-like extending OSF (hereinafter referred to as “R-OSF”) may appear in some cases depending on the pulling condition of a single crystal. At the same time, there occur, in the in-plane area of the wafer, defects that called “grown-in defects.” These grown-in defects are formed during single crystal growth and detected in wafers subjected to heat treatments or predetermined evaluation processes.
FIG. 2
schematically illustrates a generally observed relationship between the pulling rate during single crystal growth and the positions where crystal defects occur. As shown in
FIG. 2
, in a silicon single crystal grown by the CZ method, the region where R-OSF appear shrinks inward from the outer edge of the crystal as the pulling rate is decreased. Therefore, when a single crystal is grown fast, the crystal in the inner region of R-OSF expands into the whole wafer, while when a single crystal is grown slowly, the crystal in the outer region of R-OSF expands into the whole wafer.
Grown-in defects observed on a surface of a wafer are different between a rapidly grown crystal and a slowly grown crystal. In the crystal that is grown fast, i.e., in the inner region of R-OSF, defects called “laser scattering tomography defects” (they are also called as “COP” and “FPD,” and are detected by different evaluation methods, but are derived from the same kind of defect) are detected. On the other hand, in the crystal that is grown slowly, i.e., in the outer region of R-OSF, defects called “dislocation clusters” are detected.
FIG. 3
schematically illustrates an example of a typical distribution of defects observed at an in-plane position A of the crystal of
FIG. 2
previously described. This schematically shows the results of observations made through X-ray topography as to the distribution of defects of a wafer after the wafer was sliced from a single crystal immediately after growth, had Cu deposited thereon while immersed into an aqueous solution of copper nitrate, and heat-treated for 20 minutes at 900° C. That is, in the in-plane area of the wafer, R-OSF appears at a position that is about ⅔ of the outside diameter, and laser scattering tomography defects are found inside R-OSF. Further, an oxygen precipitation-promoting region exists immediately outside R-OSF so as to touch R-OSF. Oxygen precipitates easily form in this region. Around the outer edge of the wafer extends a region where dislocation clusters easily occur. Furthermore, it is observed that a denuded zone free of dislocation clusters is slightly present immediately outside the oxygen precipitation promoting region, and a denuded zone free of laser scattering tomography defects is slightly present inside R-OSF so as to touch the ring.
OSF impair electrical properties, e.g., in the form of increased leak current while showing themselves up in a high-temperature thermal oxidation process during device fabrication, and dislocation clusters also greatly deteriorate device characteristics. Therefore, a single crystal is usually produced by adjusting the growing rate so that R-OSF is located around the outer edge of a wafer. On the other hand, laser scattering tomography defects are factors for deteriorating the initial oxide film withstand voltage characteristics, and they must also be minimized.
As described earlier, to suppress the occurrence of R-OSF on a surface of a wafer, a single crystal is usually grown under such a condition that the R-OSF position is limited within the outer edge of the wafer. However, it is known that the R-OSF position is determined, in addition to the pulling rate, by the highest temperature range (from the melting point to 1250° C.) in which the crystal stays during growth, and is hence affected by the heat history of the crystal in the highest temperature range during pulling. Thus, to determine the R-OSF position, attention must be paid to two factors, i.e., the temperature gradients in the direction of the pulling shaft and the pulling rate, which are to be achieved while a single crystal being grown stays in the highest temperature range. That is, the R-OSF position can be limited around the outer edge of a wafer by decreasing the temperature gradients when the pulling rate is not changed, or by decreasing the pulling rate when the temperature gradients are not changed.
To check the position and width of R-OSF occurring in the in-plane area of a wafer, it is effective to observe the distribution of defects in the wafer through X-ray topography after immersing the wafer that is processed from an as-grown single crystal into an aqueous solution of copper nitrate to thereby deposit Cu thereon, and heat-treating it for 20 minutes at 900° C. Further, the position of the previously described oxygen precipitation-promoting region present immediately outside R-OSF can also be checked through a similar method.
When a silicon single crystal has low oxygen content of, e.g., 13×10
17
atoms/cm
3
or less, one may not observe R-OSF clearly with the a

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