Outdiffusion of Cu through Au: Comparison of (100) and (111) Cu films epitaxially deposited on Si, and effects of annealing ambients

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Abstract
Outdiffusion of Cu through thin gold films is studied as functions of Cu orientation and annealing ambients. (100) Cu films were epitaxially deposited on (100) Si, and (111) Cu on (111) Si, with commonly (111) oriented Au films deposited on Cu. Heating was conducted at 170 °C to minimize the Cu silicide formation. Structures with (111) Cu invariably give a higher surface accumulation of Cu than the counterparts of (100) Cu for each ambient, including the as-deposited samples. The accumulation is the highest in air, followed by N2-O2 and N2-H2, in agreement with the ambient effects on many other systems with gold as the overcoating layer. The orientation dependence of the outdiffusion rates of Cu through Au is related to the bonding structures at the Au-Cu interface. The face-centered-cubic structure of Cu makes the Au/Cu (111) interface more favorable for the release of Cu than the Au/Cu (100) interface, in agreement with the observed orientation dependence.