Nanoindentation assessment of aluminium metallisation; the effect of creep and pile-up

As miniaturisation of microelectronic devices continues, the metallisation connecting components has smaller and smaller dimensions, especially width and thickness. The mechanical properties of the deposited metal are very different from those of bulk material and it is important to evaluate them accurately if the reliability of the metallisation is to be optimised. The assessment of thin aluminium metallisation is possible by nanoindentation, but to extract properties useful for life prediction such as yield stress or creep relaxation behaviour, additional modelling is necessary using finite element analyses (FEA). In this study, evaporated aluminium layers from 100 to 600 nm thick on (1 0 0) silicon were indented to various depths using a loading cycle with a high load hold to assess creep performance. The load–displacement curves were then simulated by FEA and the results were compared to identify the yield and creep properties of the coating. The yield stress required to fit experimental data is reduced as the thickness of the aluminium increases. A primary creep model, such as the modified time hardening model used here, is necessary to fit the displacements in the hold period. Using the derived creep model it has been confirmed that a 60-s peak load hold is necessary to allow creep run-out in the nanoindentation testing of metallisation.