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Impact of Al in Cu alloy interconnects on electro and stress migration reliabilities
Authors:Kazuyoshi Maekawa  Kenichi Mori  Kazuhito Honda  Koyu Asai  Masayuki Kojima
Affiliation:a Production Technology Development Division, Renesas Technology Corporation, Itami, Hyogo 664-0005, Japan
b Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Abstract:The reliability of Cu interconnects was successfully improved by applying a CuAl alloy seed. However, the effect of additive Al on the reliability is not fully understood. In order to reveal the reliability improvement mechanism, Cu films using CuAl alloy seed were investigated in detail. As stress induced voiding (SIV) as well as electromigration is caused by migration of vacancies and/or Cu atoms, the measured activation energy value of electromigration using CuAl indicates that the fast diffusion paths are Cu grain boundaries. The analysis using high lateral resolution scanning type secondary ion mass spectrometry (nano-SIMS) clarifies that additive Al in ECP-Cu film is mainly localized at grain boundaries. Furthermore, positron annihilation was used to probe vacancy-type defects in Cu films. The CuAl films before recrystallization contain larger and higher density vacancy-type defects. Whereas, the recrystallized CuAl films after annealing above 250 °C contain smaller and lower density defects. Furthermore, CuAl films with annealing above 350 °C contain less Al inside the grains. These results represent that Al atoms in Cu films with annealing above 350 °C are exhausted from inside grains to the grain boundaries, and the spewed Al atoms existing at Cu grain boundary effectively prevents the diffusion of Cu and/or vacancies.
Keywords:Copper interconnects   CuAl   Reliability   Positron annihilation
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