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Microstructure variation with trench aspect ratio has been investigated in both as-deposited and annealed damascene Cu interconnect lines using Orientation Imaging Microscopy (OIM). The microstructure was characterized by measuring mean grain size, grain size and grain boundary distribution and grain aspect ratio. Since the lines were highly twinned, the mean grain size was measured both before and after neglecting the presence of Sigma3 boundaries. The mean grain size was higher for the annealed lines compared to as-deposited lines and increased proportionally with decreasing trench aspect ratio in the annealed Cu lines with higher aspect ratio. The distribution of coincident site lattice (CSL) boundaries revealed a higher fraction of Sigma3 boundaries in all the lines; their increase upon annealing in the majority of the specimens was accompanied by a decrease of Sigma9 CSL boundaries. The fraction of CSL boundaries was higher in the lines with higher aspect ratios and increased upon annealing in most of the lines. The role of pitch distance (line spacing) in influencing the mean grain size and CSL boundary distribution was identified. Additionally the microstructure was characterized by measuring the grain boundary misorientation distribution. The grain shape was characterized by measuring the grain aspect ratio. Strain distribution in the lines was examined using the Image Quality (IQ) parameter., (Copyright 2004 Elsevier Ltd.)