Exploration of Interfacial Materials Chemistry Control to Improve Cu Wire-Bonding Reliability

Abstract

Copper (Cu) wire bonding, with its advantages of higher electrical conductivity and better mechanical strength, has replaced gold wire bonding as a proven, cost-effective electrical interconnection solution for integrated circuit packaging for the past 15 y. Early Cu wire-bonding development required overcoming several technical challenges, including bond pad damage caused by copper’s hardness and brittleness relative to gold. A more chemistry-related challenge of using Cu as a bonding wire is its well-known reactivity with oxygen. An inert atmospheric envelope of forming gas surrounding bonding capillary was developed to prevent the oxidation of Cu wire during electronic flame off to enable a strong bonding. Another more elusive materials-chemistry-related reliability challenge, with a typical low ppm occurrence, has been the chloride-induced corrosion defects between the Cu wire and Al bond pad. The opportunistic low-level chloride contaminations can originate from various points of the packaging manufacturing process flow, often rendering it untrackable. In this paper, we present recent efforts to systematically control interfacial materials chemistry across Cu-bonding wire, Cu-Al bimetallic contacts, and CuxAly intermetallic compounds to eliminate corrosion defects and improve the overall bonding reliability. The prevailing manufacturing solution is to utilize Pd-coated Cu-bonding wire that can only partially mitigate the CuxAly intermetallic corrosion vulnerability. We utilized a real-time corrosion screening metrology to explore the underlying interfacial materials chemistry that drives vigorous corrosion between Cu wire and Al bond pad when exposed to a trace level of chloride contaminant. Combined with scanning electron microscope, sensitive IR spectroscopy, and electrochemical characterization, our data show that strategic surface modification on both Cu-bonding wire and exposed CuxAly intermetallic can have a significant impact on reducing corrosion defect rates. The obtained mechanistic insights provide several new strategies enabled by a novel Cu-selective passivation coating technology to effectively mitigate Cu wire-bonding corrosion defects. Implications for improving overall Cu wire-bonding reliability will be presented based on these new approaches with low-cost and packaging-friendly advantages.