Metal Surface Contamination During Phosphorus Diffusion

We present work on the impact of surface contamination before phosphorus diffusion on solar cells and life-time samples. Metal surface contamination on KOH/IPA textured Cz wafers was measured by the Sandwich-Etch ICP-MS technique. High surface contamination, especially of Cu, was found directly after texturing, which is attributed to low solubility of metals in diluted, non-oxidizing alkaline solution. Different contamination levels were reached by applying standard cleaning procedures such as HCl/HF dip sequences and the piranha etch. The emitter profiles that were tested ranged from heavy diffusions of 45 Ω/sq to shallow diffusions of 120 Ω/sq. It was found that threshold values which obviously impact life-times and solar cell performance (Voc) were much higher than expected.