The study of grain growth in copper interconnects

Reliable quantification of microstructural parameters via microscopy is of primary importance for the quantization of materials properties, especially for Cu lines where Cu is used as an interconnect material for Integrated Circuits (IC). The microstructure of inlaid Cu lines has been investigated using transmission electron microscopy (TEM) as a function of annealing conditions, effective linewidth and time. A grain-size analysis was carried out for the purpose of studying the effects of annealing copper lines of effective linewidths of 0.08, 0.12, 0.16, 0.25, 0.50, and 0.75 μm at 350°C for 6, 12 and 24 hours respectively. The grain size distributions were observed to be lognormal. Unlike the conventional belief, the results obtained from the grain-size analysis reveal that the rate of grain growth rapidly decreases with increasing effective width of the Cu trenches. Several possible mechanisms for this phenomenon are examined. An explanation is offered for the observation that interface diffusion occurs in addition to bulk diffusion in the narrow trenches.