Texture Evolution of Abnormal Grains with Post-Deposition Annealing Temperature in Nanocrystalline Cu Thin Films
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E annealed at temperatures below those needed to stimulate grain growth in coarse-grained counterparts, or even at room temperature, nanocrystalline materials undergo grain growth, often in an abnormal manner.[1–5] Abnormal grain growth (AGG) and the accompanying texture evolution in nanocrystalline materials would influence their mechanical properties. In this regard, understanding of the abnormal grain growth-related texture evolution is directly connected to the development of new nanoelectronic devices with excellent mechanical properties. However, research on orientations of abnormally grown grains (briefly, abnormal grains, hereafter) within nanocrystalline matrices has been done restrictively, possibly because sizes of abnormal grains are still small, in some cases, even for the transmission electron microscopy (TEM) selected area diffraction pattern (SADP) method. Grain growth in thin films has been modeled on the basis of equibiaxial stress condition[6,7]: The h111i fiber SUNG BO LEE, Research Scientist, SEONG-HYEON HONG, Professor, and DONG NYUNG LEE, Emeritus Professor, are with the Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, South Korea. Contact e-mail: [email protected] DONG-IK KIM, Research Scientist, is with the Korea Institute of Science and Technology, Seoul 136-791, South Korea. Manuscript submitted August 16, 2012. Article published online November 27, 2012 152—VOLUME 44A, JANUARY 2013
texture is driven by surface energy minimization and h100i by elastic strain energy minimization. And, such textures accompanying AGG have been reported for metal films with initial grain sizes of the nanocrystalline scale (including submicrometer)[1,2,4] and explained in terms of surface energy minimization or strain energy minimization. However, in some cases, other types of AGG textures have been reported. For example, Longworth and Thompson[8] observed that in h111i-textured matrix Al alloy thin films, strong h112i and weak h110i texture components develop during AGG. Such textures are not explained simply by surface energy minimization and strain energy minimization. Eventually, we aim to suggest a comprehensive model for the annealing texture evolution in thin films. The annealing texture is classified into the (abnormal) grain growth and recrystallization textures. The present study deals with nanocrystalline Cu films, which have little dislocation density, and in this case recrystallization driven by the reduction of dislocation density is not likely to occur. Thus, this work concentrates on the grain growth texture evolution. If films contain a high density of dislocations, they could exhibit the recrystallization texture during annealing, which is outside the scope of the present study. Readers interested in the topic should refer to a review article by Lee.[9] In this work, we cover texture analysis of abnormal grains in nanocrystalline Cu films using a novel automatic orientation and phase mapping technique for TEM. Especially,
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