Comprehensive intermetallic compound phase analysis and its thermal evolution at Cu wirebond interface

Intermetallic compound (IMC) formation in Cu wire bond is of great technological importance since it underpins the bonding characteristics between Cu wire ball and Al pad, which are directly associated with long-term reliability. Thus, extensive works have recently been performed 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 (EPMA) to identify IMC phases. However, to understand IMC growth and phase evolution in depth, transmission electron microscopy (TEM)/nano-beam electron diffraction (NBED) study is necessary. We have recently performed TEM/NBED investigation on 20 μm (=0.8 mil) Cu wire bond interface and unambiguously verified the presence of metastable θ'-CuAl2 IMC phase as the majority IMC phase. In this study, further comprehensive IMC phase analyses were performed for side-by-side comparative study between 18 μm (=0.7 mil) Pd-coated Cu wire and 18 μm Pd-coated Cu wire that was followed by Au flash coating using SEM and TEM/NBED in conjunction with structure factor (SF) calculation. For SEM examination, cross-section of Cu wirebond samples was prepared by a combination of mechanical polishing and Ar ion cross-section polisher. TEM sample was prepared using dual-beam focused ion beam technique. Cross-sectional SEM result shows that in both of the wirebonds, discontinuous IMC patches were formed at the Cu/Al interface immediately after packaging, and they grow toward Al pad after HTS test at 150 °C for 1000 h. TEM/NBED result combined with SF calculation revealed that in both of the wirebonds, the majority of IMC phase was metastable 9'-CuAl2 phase while Cu9Al4 phase was also found occasionally between metastable 9'-CuAl2 pha- e and Cu wire in as-packaged wirebonds. After HTS test, both of the phases grow in size. SEM, and TEM/NBED results suggest that there is no Au coating effects in terms of IMC morphology, phase formation and their thermal evolution. Based on NBED and EDS results, possible Cu9Al4 growth mechanism will be discussed.