Growth and reactivity of Al-Cu intermetallic compounds under ideal conditions

Cu wire bonding is fast coming up as Au wire replacement in semiconductor industry due to its lower price and several expected technological advantages. Cu bond wire reliability on Al pads is an ongoing study of interest and one of the mostly addressed topics is the degradation of Al-Cu intermetallic compounds (IMC) under high temperature or humidity stress conditions. The majority of published studies were carried out on Al-Cu systems in encapsulated packages. Many of them reported the degradation of the wire/pad-interface by cracks which are starting from the ball periphery and are penetrating inwards. Some also report about voids which are forming within IMCs or at interfaces. Questions have been raised about the root cause of the reported physical defects and the associated reliability risks. This paper is a more fundamental research investigating the Al and Cu diffusion reactions and the formation of individual IMC phases at clean and ideal Al/Cu interfaces without potential impacts of bond process and molding compound. It is conducted by using thin film couples of clean sputtered Al and Cu layers, which have been annealed under forming gas atmosphere at different temperatures (150°C-275°C) and for various time periods. Two different thin film couples have been used: a) 5000 nm Al followed by several μm copper to simulate an almost infinite Al reservoir and b) 650 nm Al followed by several μm Cu to simulate a typical Al reservoir after bonding. The samples were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Focused Ion Beam (FIB) and X-ray diffraction (XRD).