Specification of [C], [O] and [N] in Solar-Grade Silicon Feedstock to be Used in DS Furnaces

An experiment was performed where UMG-Si and modified Siemens Si were mixed (20/80) and compared in [C], [O], and [N] before and after directional solidification [15], using silicon-nitride-coated quartz crucibles. SIMS was used for the measurements. (Note that the modified Siemens Si was thought by the manufacturer to be 11N but SIMS detected S at high enough level to make the material 7N). A control of the modified Siemens Si was included before and after the directional solidification. A schematic of the experiment is shown in Figure 6.7 and the SIMS data are in Table 6.17.

The conclusions of the experiment were:

(A) The directional solidification process itself (with the nitride liner) dominates the C, O and N levels in the mc-Si solar wafers, in good agreement with the Olsen and 0verlid [5] results discussed before.

(B) Specifications for very low levels of C, O and N in PV Si feedstock are not warranted if a directional solidification process with nitride liner is used.

(C) The opportunity to reduce C, O and N in mc-Si wafers is to focus on the directional solidification process, not the feedstock.

Table 6.17 SIMS impurity concentrations (x1016 atoms cm 3) in PVSi feedstock.

Carbon

Oxygen

Nitrogen

7N Si feedstock

<0.5

<1.0

0.03

5N Si feedstock

37

200

0.55

7N corner

55

11

1.3

7N center

71

8.0

1.1

7N edge

54

7.8

0.96

7N:5N corner

41

16

1.1

7N:5N center

49

18

1.4

7N:5N edge

49

13

1.2

Typical SIMS detection limit

0.5

1.0

0.02

PV Si wafer specification (SEMI M6 [2])

<100

<80

No spec