G.R. Mount, L. Wang, K. Putyera, M. Lepage
Evans Analytical Group, US
silicon, contamination, characterization, measurement, SIMS, NAA, ICPMS, GDMS
The term ‘Solar Grade Silicon (SoG)’ has been used for many years but until recently, there were no specifications as to what this actually meant. Without specifications, suppliers of silicon have been using ‘number of nines’ purity as one way to differentiate their product. However the number of ‘N’s can be adjusted to almost any purity by choosing which elements to include and exclude in the measurement. More importantly ‘number of nines’ does not differentiate between elements that are important for solar performance and those that are not. The SEMI organization has recently released specifications for virgin silicon feedstock material (Solar Grade Silicon) defining 4 grades . The specifications define upper limits of impurities for various elements, grouped by their function and importance on PV performance. There is no discussion of ‘nines’. In this work we show various methods for contaminant analysis including SIMS, ICP-MS, GDMS and NAA. Some of these are official SEMI test methods for PV silicon; SIMS (PV25-1011), GDMS (PV1-0211) and NAA (PV10-1110). We report on the strengths and limitations of each.