Electron microscopy study on intermetallic compound formation in Cu-Al bond interface

Due to the importance of fundamental bonding mechanism associated with intermetallic compound (IMC) formation in Cu wire bond, extensive studies have recently been made to understand IMC formation at the interface between Cu wire ball and Al pad metallization. Most of the previous works implemented qualitative scanning electron microscopy (SEM)-based analysis techniques such as energy dispersive x-ray spectroscopy (EDS) and electron probe micro-analyzer to identify IMC phases. To understand IMC growth and phase evolution in depth the present investigation systematically uses transmission electron microscopy (TEM)/electron diffraction (ED), EDS and structure factor (SF) simulation for a comprehensive study. A IMC phase analysis was performed for Cu wire bonds using SEM and TEM/nano-beam ED (NBED) combined with SF simulation. For SEM examination, cross-section of Cu wire bond samples was prepared by a combination of mechanical and Ar ion polishing techniques. TEM samples were prepared using dual-beam focused ion beam technique. SEM result showed that discrete IMC patches were initially formed at the Cu/Al interface and they spread across the Cu/Al interface during high temperature storage (HTS) environment at 150 °C for 1000 hours. At the same time the IMC thickens growing towards the Al pad. TEM, NBED, and EDS results combined with SF calculation revealed the evidence of metastable θ ´-CuAl₂ IMC phase formation at Cu/Al interfaces before the HTS test. This θ ´-CuAl₂ IMC phase grew in size after HTS test. Possible reasons for the presence of the metastable CuAl₂ phase are discussed.