Silicon Refining Via Plasma Processes

A plasma process for the purification of silicon was proposed, tested and used by the company Kawasaki more than 10 years ago, and demonstrated as a valuable tool for

Table 2.13 Partial pressures (atm) ofatomic and molecular components during thermal plasma decomposition of SiO2 (after A. Fridman, ref. 126, page 433).

Components

T =3000 K

T=4000 K

T=5000 K

T = 6000 K

O

1.02 x 10-4

1.36 x 10-2

1.44 x 10-1

4.4 x 10-1

Si

4.72 x 10-4

1.42 x 10-2

1.44 x 10-1

4.05 x 10-1

SiO

9.99 x 10-1

9.7 x 10-1

7.12 x 10-1

1.9 x 10-1

SiO2

3.66 x 10-4

4.67 x 10-4

2.7 x 10-4

2.67 x 10-5

O2

8.34 x 10-7

8.4 x 10-5

4.34 x 10-4

4.23 x 10-4

SiO (molar fraction)

99.9%

98.5%

83.1%

31.5%

Table 2.14 Impurities content of Si in the different stages of the Photosil Process.

Element

MG-Sia

UMG1b

Wafers c

Wafers3

Photosil-Sib

Total concentration

50-110

5-30

(ppmw)

B

7-15

7-8

2-6

0.85-0.7

~0.3

P

24-40

10-12

4-15

2.2-3

~1.0

Al

80-90

30

0.5-5

0.012-0.02

<2

Ca

350-450

2-8

Fe

30-90

150

<1

<1.6

<2

Ti

10-20

<2

Cu

<2

aafter Degoulange [50], bafter ref. 124 optimized material, cafter Alemany et al. [131]

removing B, and P within the French Photosil Project [129]. The rationale behind the process is the formation of volatile metallic oxides (and oxyhydrates) in a plasma of argon and water, at a temperature higher than the boiling point of silicon (2027°C). At this temperature the process is much more efficient than at the temperatures 1410-1700°C, which are the process temperatures applied in Khattack’s [65, 66] experiments and in the Timminco-Becancour process [81].

The combination of an inductive plasma torch and electromagnetic stirring of molten silicon has been used to refine MG-Si, as in the final purification step of the Photosil Process [48, 70, 130] under the addition of reactive gases (O2 and H2). By this process, where the electromagnetic stirring facilitates the continuous renewal of the melt surface, on which the plasmochemical reaction takes place, a significant reduction of the con­taminants (B, C, P, Al, Ca and Fe) concentration is obtained, before further reduction by a directional solidification step. The results are reported in Table 2.14

It turns out that the process works with a reasonable efficiency for the removal of B and P, when the gas mixture consists of hydrogen and oxygen, leading to final values quite acceptable for a solar silicon feedstock. Concerning the other impurities, the segregation effect during the solidification seems to prevail, as the concentration drop is consistent for segregation coefficients around 10-2.

Stabilized efficiencies of the best plasma-purified UMG-CZ grown silicon solar cells were reported to range slightly above 17% [124] against a value around 16% for 100% Photosil multicrystalline and 10% for a directionally solidified UMG silicon.