Effect of the substrate bias voltage on the physical characteristics of copper films deposited by microwave plasma-assisted sputtering technique

Pure copper films have been deposited by microwave plasma-assisted sputtering on (100)-oriented single crystal silicon substrates mounted on a water-cooled substrate holder either maintained at the floating potential or biased to various dc voltages ranging from 0 to -125 V. The argon pressure was fixed at 0.13 Pa and argon ions from the discharge produced in a distributed electron cyclotron resonance microwave plasma chamber were used for sputtering of a copper target biased to -600 V. The crystallographic structure and surface morphology of copper films were determined by x-ray diffraction techniques and atomic force microscopy, respectively. The magnitude of residual stresses developed in these films was calculated from the radius of curvature of Si substrates. The electrical resistivity of films was deduced from the thickness and sheet resistance of films determined by profilometry and four point probe measurements. The evolution of the deposition rate, surface roughness, microstructure, magnitude of residual stresses, and electrical resistivity of films was investigated as a function of the substrate bias voltage. The major characteristics of copper films were found to vary significantly as the negative substrate bias voltage increased from -40 to -125 V or with increasing argon ion energy. The resputtering process of a fraction of copper atoms by energetic incident argon ions was responsible for the modification of characteristics of films deposited on biased substrates. The quality of copper films in terms of surface morphology and electrical resistivity in particular was found to be excellent for incident argon ion energy values lower than 70 eV