12. Wafer Cleaning, Etching, and Texturization

Wafer preparation for silicon PV includes wet chemical cleaning, etching, and texturization steps. Aqueous solutions containing either acids or strong bases result in very different etch rates. Underlying chemistry is used for all three applications. Typical cleaning mixtures such as RCA-SC1 and RCA-SC2, SPM, and dHF are introduced with their respective properties as well as acidic etching systems like hydrofluoric acid/nitric acid (HF/HNO₃) and alkaline mixtures such as potassium hydroxide/isopropanol (KOH/IPA). While the latter is used for texturing monocrystalline wafers due to the anisotropic etching behavior, the former HF-containing systems are generally used for isotropically texturing multicrystalline silicon wafers. Fundamental chemical, physical, thermodynamic, and kinetic aspects of these systems are presented and discussed. In all cases, it has to be pointed out that a complete understanding of the reaction mechanisms causing the observed properties is still missing to a large extent. Therefore, many aspects of silicon cleaning, etching, and texturization have only been optimized empirically. Further studies are necessary to provide a basis for future improvements, which are not only focused on scientific aspects but also on environmental and economic issues.