Grain size distributions of 100 nm wide Cu interconnects have been evaluated with a nondestructive method utilizing X-ray diffraction. The log-normal size distributions were determined from area average and volume average column lengths assuming spherical grains. Median grain size and size distributions obtained by this method were confirmed to be in good agreement with those from TEM. It was also found that lower limit of Cu interconnects to evaluate grain sizes was 1 x 10(-5) g in a 10 mm(2) chip. This lower limit allows evaluation of grain sizes in interconnects with less than 50 nm line width for future ULSIs. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3556985] All rights reserved.