Multi-stage arsenic doping process to achieve low...

Details Multi-stage arsenic doping process to achieve low... Multi-stage arsenic doping process to achieve low...

2000-05-11

2001-11-06

Kunemund, Robert (Department: 1765)

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

Processes of growth from liquid or supercritical state

Having pulling during growth

C117S014000, C117S017000, C117S019000, C117S932000

Reexamination Certificate

active

06312517

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a multi-stage arsenic doping process to achieve low resistivity in silicon crystal grown by Czochralski method.
Arsenic is an ideal dopant used to achieve lower resistivity in silicon crystals grown by Czochralski process because of its high solubility in silicon. However arsenic is highly volatiles at temperature higher than 617° C. The Surface of the silicon melt: is at its melting point (1412° C.) or higher. In popular practice, arsenic is fed into the melt from a feed hopper located a few feet. above the melt level. However, due to higher temperatures, loss of arsenic to surrounding environment (argon gas) is violent, which results in generation of particles. Generation of oxide- particles (sub-oxides) is enhanced by local reduction in temperature caused by sublimation of arsenic. These particles can act as heterogeneous nucleation sites and often result in failure of the crystal pulling process. Excessive generation of particles does not allow sufficient doping of arsenic to achieve higher arsenic concentrations in the melt required to achieve lower resistivity targets.
SUMMARY OF THE INVENTION
The present invention overcomes the above described difficulties and disadvantages associated with such prior art processes by introducing arsenic in multiple stages such that particle generation per stage is decreased whereas the particle removal capacity of the crystal puller, which is a strong function of argon flow rate, remains practically unchanged. For example the following mass balance in the continuum sense must hold:
Rate of particle accumulation=Rate of particle generation—Rate of particle removal
ⅆ
n
ⅆ
t
=
r
g
-
r
r
(
1
)
where n is number of particles in the crystal puller, t is time (s), r
g
is the rate of generation of particles (number/s) and r
r
is the rate of particle removal (number/s) Also, in the continuum sense total number of particles is given by following equality:
Number of particles=Number of particles initially present+Integral of difference between rate of particle generation and rate of particle removal
n
=
n
0
+
∫
0
t
⁢
ⅆ
n
ⅆ
t
⁢
 
⁢
ⅆ
t
=
n
0
+
∫
0
t
⁢
(
r
g
-
r
r
)
⁢
 
⁢
ⅆ
t
=
n
0
+
∫
0
t
⁢
F
⁡
(
feedrate
,
argonflowrate
)
⁢
 
⁢
ⅆ
t
(
2
)
where
0
is the initial number of particles present and F(feed rate, argon flow rate) is a function of arsenic feed rate and argon flow rate. Equation (2) indicates that as the arsenic feed rate decreases the total particle count in the crystal puller decreases. A continuous process with a lower arsenic feed rate can be closely mimicked by a multi-stage process in which arsenic is fed in multiple stages. In a single-stage process total amount of arsenic is fed at once. In such a process rate of particle generation is very high such that rate of particle removal cannot match the rate of particle generation and, hence, the number of particles in the crystal puller increases. In a multi-staged process the amount of arsenic fed per stage decreases by a factor of the number of stages. Thus, in a multi-staged process the rate of particle generation is decreased and the total number of particles in the crystal puller is lower. Thus, the possibility of process failures due to particle related dislocations or loss of crystal structures is greatly reduced.
Other objects and features will be in part apparent and in part pointed out hereinafter.


REFERENCES:
patent: 4980015 (1990-12-01), Ono et al.
patent: 5242531 (1993-09-01), Klingshirn et al.
patent: 0 635 588 A1 (1995-01-01), None
patent: 0 635 588 B1 (1995-01-01), None
patent: 9227275 A (1997-09-01), None
patent: WO 97/36024 (1997-10-01), None
patent: WO 98/35074 (1998-08-01), None

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