On the physics of thick copper film deposition by an innovative thermal evaporation process

Steel industry and film deposition technologies are more and more combined to bring innovative and high value added products. The study presents the development of a copper deposition technique based on the evaporation of the metal at elevated temperature (1500 °C) in a high vacuum environment. Such extreme conditions require the complete understanding and control of the experimental setup by modelling the thermal transfers that occur during the evaporation. This process involves the improvement of the deposition efficiency by increasing the directionality of the evaporating gas flow owing to the use of a de Laval nozzle at the top of the evaporator. This innovative deposition technique allows the deposition of copper thick films exhibiting equiaxed grains. Characterisation by scanning electron microscopy, electron-backscattering diffraction and transmission electron microscopy highlight that grain size ranges from 50 to 400 nm. This fine microstructure is inferred to be related to the large oxygen content measured in the films. Furthermore, a large number of twinned grains are observed with a small twin spacing. Since the films are deposited at high deposition rate and low substrate temperature, large quantities of voids are trapped during the film formation. Positron Annihilation Spectroscopy is used to analyse the type of point defects. The preliminary results suggest that the films present a particular type of point defect in which the oxygen solute atoms could be trapped. Finally, nanoindentation revealed large levels of hardness for the films. It is expected that this hardening is a consequence of the small grain size, the large number of twins and the solid solution hardening due to the presence of oxygen.