Metastable Pitting Corrosion of Aluminum, Al-Cu, and Al-Si Thin Films in Dilute HF Solutions and its Relevancy to the Processing of Integrated Circuit Interconnections

Abstract

ABSTRACTWe have investigated the pitting corrosion of Al, Al-Cu, and Al-Si thin films in dilute hydrofluoric acid solution. Additions of Cu and, to a lesser extent, Si cause an increase in open circuit potentials (OCP), pit densities and pit sizes as compared to pure Al. Pits in Al-Cu alloys are associated with Cu depleted grain boundaries which are galvanically coupled to adjacent θ-phase precipitates. Pits formed in Al-Si alloys are associated with the Al matrix which is galvanically coupled to Si nodules. θ-Al2Cu has different electrochemical characteristics than Al even though both maintain a thin Al surface oxide. θ-Al2Cu has a more positive OCP, above the average pit repassivation potential for pure Al. Since θ-Al2Cu supports cathodic reactions at enhanced rates compared to pure Al, its presence raises the potential of the adjacent pure Al grain boundary to values near its average breakdown potential and above its average repassivation potential. Metastable pitting is thus promoted.