Optimized HF solutions for ultra-clean Si surfaces

HF based solutions are widely used in IC manufacturing industry, e.g. for etching, patterning and cleaning. However, trace amounts of nobel metal ions, such as Cu, Ag or Au ions, present in the HF solution can deposit on the Si surface. This metallic contamination destroys the chemical stability of the Si surface and results in a reduced production yield. A more fundamental understanding of the outplating behaviour of impurities from HF solutions is therefore necessary. In this study the Cu outplating on Si surfaces is investigated in detail. Since mobile charge carriers are involved in the Cu deposition process, Si semiconductor properties play an important role. This is demonstrated by evaluating the effect of illumination of the Si surface on the Cu outplating behaviour. Also the effect of HCl addition to HF solutions is investigated. A model is presented to explain the observed phenomena and various strategies to reduce the metal outplating from HF solutions are illustrated.