Unintended Catalysis and Defect Formation in Electroless Metallization of Semiconductor Wafers

Abstract

Owing to the advantages like minimized material waste and green streamlined process, additive metallization has grown as an attractive technique in semiconductor fabrication. Still, reliability issues such as the formation of metallic nodules prevent its widespread use. To date, the formation mechanism of these metallic nodules remains unclear. In this study, a comprehensive formation mechanism of metallic nodules on electroless-plated wafers was investigated using a thorough physical failure analysis (PFA). Optical microscopy, field-emission scanning electron microscopy (FESEM), focused ion beam (FIB) cross-sectioning, and energy-dispersive X-ray spectroscopy (EDX) were among the analytic techniques used to reveal that the nodules (mainly composed of Ni and Pd) are loosely adhered to the passivation layer. Metal oxide residues were identified as potential catalytic sites for unintended metal deposition by EDX line scan after the delayering process. A formation mechanism of nodules is proposed, which includes the leftover of metal residue during the physical vapour deposition (PVD) etching process, oxidation of metal residue and autocatalytic deposition using electroless plating. A potential mitigation strategy, which is ultrasonic, was proven to be effective in dislodging the nodules upon formation. These results provide valuable insight into preventing defect formation and streamlining the electroless metallization for cutting-edge semiconductor applications.