The mechanical property and microstructural thermal stability of gradient-microstructured nanotwinned copper films electrodeposited on the highly (111)-orientated substrates

Three different substrates with varying (111) textures were used to study the epitaxial growth behavior of nanotwinned copper (nt-Cu) films during direct current electroplating. The (111) proportions for substrates are 78.5%, 97.5%, and 100%, while for the electrodeposits they are 69.3%, 92.7%, and 98.3%, showing a positive correlation between the (111) proportions of the substrates and the electrodeposits. Secondary ion microscopy (SIM) reveals a transition layer consisting of fine grains forms between the columnar nanotwinned grains region and the substrate, exhibiting a gradient microstructure within the nt-Cu films. The thickness of the transition layer shows a reducing tendency as the (111) proportion of substrate increases. Furthermore, to investigate the mechanical properties of the transition layer, a nano-indentation test was performed on the cross-section of gradient-microstructured films. The average hardness of the transition layer near the substrate is 3-4 GPa, which is approximately two times higher than that of the columnar nanotwinned region. The excellent hardness is probably induced by the decrease in grain size and the presence of high-density twin boundaries within the grains. An in-situ annealing test at 300 degrees C was performed on these three electrodeposits to assess their microstructural thermal stability. It is noticed that the thickness of transition layer would affect the competition of grain growth between substrate grains and columnar nanotwinned grains during aging. Higher microstructural thermal stability could be achieved with fewer grains in the transition layer. Overall, this study reveals the high mechanical properties and microstructural thermal stability of the transition layer, which should be helpful in understanding and promoting the application of nt-Cu thin films.