Identifying and Mapping Impurity Sources in Deposition-Prone Chemicals for Wafer Surface Wet Processing

Abstract

Contaminant levels in chemicals used for wet processing are critical to solar cell performance. During wafer immersion in chemical and DI water baths, a thin film forms on the surface, trapping impurities. Also, recycled polysilicon for ingot and wafer manufacturing utilizes similar chemicals for etching, which are again prone to contamination. Contaminants often enter during chemical handling, which is outside the defined boundary of cell and ingot manufacturing area. Despite multiple purification steps, impurities persist even in electronic or photovoltaic grade chemicals. Key chemicals such as HCl, HF, H₂O₂, HNO₃, KOH, and DI water were analysed using ICP-MS to trace elemental impurities, often linked to raw materials (e.g., As and K from fluorspar) or contact with construction materials (e.g., Fe, Mn, and Cr from mild steel piping). Impurities adhering to wafer surfaces, particularly Fe, can reach concentrations up to 10¹¹ atoms/cm², significantly affecting carrier lifetime and cell efficiency. The Landau-Levich equation indicates that higher withdrawal velocities result in thicker films, increasing impurity deposition rates. To mitigate these detrimental effects, impurities must be eliminated at the source, and tolerance limits should be established based on their deposition susceptibility through the liquid film. This study quantifies impurities that may be carried forward in wafer and cell processing, ultimately hindering solar cell functionality. It underscores the necessity of controlling contamination sources in chemical processing to enhance solar cell efficiency and lifetime.