Stability of Cu-islands Formed on Si Substrate via "Dewetting" under Subsequent Thermal Cycling.

Very thin metallic films deposited on a substrate often dewet upon thermal exposure, forming discrete islands of micrometer and nanometer-sized metal particles. Herein, Cu islands on Si substrate, which were formed due to agglomeration (or "dewetting") of Cu thin film at 600 °C, were exposed to thermal cycling and the ensuing evolution in their morphology was monitored. Thermal cycling was performed between either -25 and 150 °C or 25 and 400 °C, using differ-ent heating and cooling rates. With faster heating-cooling rates, a change in the shape and size of the Cu islands was observed, whereas a slow heating-cooling rate did not induce any noticeable effect on their morphology. Furthermore, the formation of new nano- and micro-sized particles, probably through the dewetting of the ultra-thin layer of Cu that was left intact during the initial agglomeration treatment, was observed during the thermal cycling performed at fast rates up to 400 °C. Finite element analysis (FEA), incorporating Anand's viscoplasticity model, revealed the existence of high strain energy density in the vicinity of the particle-Si interface when the thermal cycling is carried at a faster ramp rate, suggesting the pivotal role of thermal stresses, in addition to the maximum temperature, in controlling the morphology of the Cu particles and dewetting of the residual ultra-thin layer of Cu on Si.