102
S. Leu and D. Sontag
original diameter of the seed)—this prevents the propagation of dislocations. Then,
in the next step, the pulling speed is reduced to form the shoulder.
The crystal is gently pulled out of the melt; the pulling speed (~0.5 mm/min) is
automatically controlled and adjusted to grow the ingot diameter to the desired value
(typically 200–300 mm). Despite this limitation the shoulder (see Fig. 5.2) can grow
quickly and the ingot reaches rapidly the desired diameter. The necking process must
be carried out carefully, because the situation must be avoided in which the thin inner
rod breaks or dislocations occur in the crystal lattice. A flat shoulder is advantageous
for production reasons. There is less waste and the entire drawing process is about
15–25% faster. However, there is a risk that, with flat shoulders, dislocations occur.
Dislocations can occur because the temperature difference between the inside of the
ingot and the edge area is too high, due to impurities or due to external vibrations.
Usually, crystals for photovoltaic solar cells are pulled in the {100} plane. If dislocations occur, they propagate by sliding over the four sides of the {111} planes and,
thus, slip outwards. Dislocations extend over a length approximately equal to the
diameter of the ingot. Afterwards they disappear.
As soon as the diameter of the ingot is reached (200–300 mm), the drawing speed
is increased to approximately 8 mm/min. The drawing speeds and the temperatures
are continuously controlled and monitored, so that a constant diameter is formed.
After the ingot has been pulled, it must be mechanically secured (using a locking
system) and slowly cooled, so that no cracks are created due to stress. The end of the
drawing process must not be abrupt—because this can trigger thermal shocks, which
can lead to dislocations in the crystal lattice. Therefore, the end of the drawing process
is initiated via a tapered tail. Incidentally, this is also where most of the impurities
are found, so the act of pulling the crystal can also purify it. Using the Czochralski
method, round ingots of up to 4 m in length, typically 2.5 m, and up to 300 mm
(12 in.) in diameter are drawn. The filling of the quartz glass crucible is about 150 kg
and can be increased with subsequent recharging to about 200 kg.
The disadvantages of the method are: (1) the wall of the quartz glass crucible can
react with the silicon, which limits the resistivity by the penetration of impurities;
(2) even if the crucible is coated with Si 3 N 4 , impurities from the coating layer can
penetrate the silicon melt. This coating acts as a barrier and the impurities entering
the silicon melt from the crucible are greatly reduced. Nevertheless the coating itself
is a source of contamination. However, the total contamination (crucible and coating)
is reduced; (3) finally oxygen from the quartz glass crucible wall penetrates into the
silicon, forming SiO 2 according to the simplified reaction:
SiO 2quartz glass crucible + 2Si melt → Si + 2SiO
(5.6)
This is, in fact, inevitable. In a typical ingot, the concentration of interstitial
oxygen is between 10
17 and 10
18 cm
−3 . Because silicon has about 10
23 atoms per
cubic centimetre, oxygen contamination is typically between 0.1 and 1 ppm.
7
7 «ppm» means «parts per million».
S. Leu and D. Sontag
original diameter of the seed)—this prevents the propagation of dislocations. Then,
in the next step, the pulling speed is reduced to form the shoulder.
The crystal is gently pulled out of the melt; the pulling speed (~0.5 mm/min) is
automatically controlled and adjusted to grow the ingot diameter to the desired value
(typically 200–300 mm). Despite this limitation the shoulder (see Fig. 5.2) can grow
quickly and the ingot reaches rapidly the desired diameter. The necking process must
be carried out carefully, because the situation must be avoided in which the thin inner
rod breaks or dislocations occur in the crystal lattice. A flat shoulder is advantageous
for production reasons. There is less waste and the entire drawing process is about
15–25% faster. However, there is a risk that, with flat shoulders, dislocations occur.
Dislocations can occur because the temperature difference between the inside of the
ingot and the edge area is too high, due to impurities or due to external vibrations.
Usually, crystals for photovoltaic solar cells are pulled in the {100} plane. If dislocations occur, they propagate by sliding over the four sides of the {111} planes and,
thus, slip outwards. Dislocations extend over a length approximately equal to the
diameter of the ingot. Afterwards they disappear.
As soon as the diameter of the ingot is reached (200–300 mm), the drawing speed
is increased to approximately 8 mm/min. The drawing speeds and the temperatures
are continuously controlled and monitored, so that a constant diameter is formed.
After the ingot has been pulled, it must be mechanically secured (using a locking
system) and slowly cooled, so that no cracks are created due to stress. The end of the
drawing process must not be abrupt—because this can trigger thermal shocks, which
can lead to dislocations in the crystal lattice. Therefore, the end of the drawing process
is initiated via a tapered tail. Incidentally, this is also where most of the impurities
are found, so the act of pulling the crystal can also purify it. Using the Czochralski
method, round ingots of up to 4 m in length, typically 2.5 m, and up to 300 mm
(12 in.) in diameter are drawn. The filling of the quartz glass crucible is about 150 kg
and can be increased with subsequent recharging to about 200 kg.
The disadvantages of the method are: (1) the wall of the quartz glass crucible can
react with the silicon, which limits the resistivity by the penetration of impurities;
(2) even if the crucible is coated with Si 3 N 4 , impurities from the coating layer can
penetrate the silicon melt. This coating acts as a barrier and the impurities entering
the silicon melt from the crucible are greatly reduced. Nevertheless the coating itself
is a source of contamination. However, the total contamination (crucible and coating)
is reduced; (3) finally oxygen from the quartz glass crucible wall penetrates into the
silicon, forming SiO 2 according to the simplified reaction:
SiO 2quartz glass crucible + 2Si melt → Si + 2SiO
(5.6)
This is, in fact, inevitable. In a typical ingot, the concentration of interstitial
oxygen is between 10
17 and 10
18 cm
−3 . Because silicon has about 10
23 atoms per
cubic centimetre, oxygen contamination is typically between 0.1 and 1 ppm.
7
7 «ppm» means «parts per million».