Influence of Bias Voltage on Copper Nitride Films Deposited by Reactive Sputtering

This paper investigates the effect of negative bias voltage applied to substrates on the structure and electrical resistivity of films deposited with rf sputtering on a copper target in reactive Ar-N₂ mixtures. To show the influence of the nitrogen content in the film, four different nitrogen flow rates have been chosen from the results of a previous study. Whatever the nitrogen flow rate when a negative bias voltage is applied to the substrates, copper diffraction peaks are detected. This is due to nitrogen resputtering during the film growth. This resputtering phenomenon can be so significant that films deposited with a low nitrogen flow rate and high bias voltage are composed only of copper. With other deposition conditions, the films are biphased: Cu + Cu₃N. In these films, the Cu₃N crystal orientation is also dependent on the bias voltage. Since the nitrogen content of the films evolves and since copper crystals are formed, the electrical resistivity of the films decreases when a bias voltage is applied. However, the electrical resistivity of biphased coatings does not reach that of copper and therefore, copper grains do not percolate in these films.