In situ TEM study of grain growth in nanocrystalline copper thin films.

Nanocrystalline metals demonstrate a range of fascinating properties, including high levels of mechanical strength. However, as these materials are exposed to high temperatures, it is critical to determine the grain size evolution, as this process can drastically change the mechanical properties. In this work, nanocrystalline sputtered Cu thin films with 43 +/- 2 nm grain size were produced by dc-magnetron sputtering. Specimens were subsequently annealed in situ in a transmission electron microscope at 100, 300 and 500 degrees C. Not only was grain growth more evident at 500 degrees C but also the fraction of twins found. An analysis of grain growth kinetics revealed a time exponent of 3 and activation energy of 35 kJ mol(-1). This value is explained by the high energy stored in the form of dislocation, grain boundaries and twin boundaries existing in nanocrystalline copper, as well as the high probability for atoms to move across grains in nanocrystalline materials.

The effect of nitrogen on the formation of nanocrystalline copper thin films.

The current success of nanocrystalline materials is due to their unusual and promising properties compared to coarser grain size materials. However, maintaining the nanocrystalline character during processes or applications is not an easy task, due to the tendency towards grain growth exhibited by nanocrystalline materials. It is well known that the addition of solutes with a strong affinity for grain boundary segregation can act as pinning centers and inhibit grain growth, particularly during the manufacturing process. However, the ideal is to use these elements/compounds only during manufacturing, and after that these elements must disappear in order to attain the desirable properties. The aim of this study is to produce nanocrystalline Cu-based thin films through controlled addition of nitrogen to inhibit grain growth. A detailed chemical composition, structural and grain size analysis of these thin films was made by Electron Probe Microanalysis (EPMA), X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The results indicate that introduction of nitrogen, even in small amounts, leads to a significant decrease in grain size, particularly if Cu3N is not yielded in the thin film during the deposition process.