Electrical characterization of a dc secondary discharge created during plasma sputtering deposition of palladium thin films

Good quality Pd thin films (for catalysis application) can be obtained by a low-pressure plasma sputtering method. The metal atom source is a helicoidal wire which is negatively biased so that argon ions created in a high frequency plasma are attracted and gain sufficient energy to induce sputtering. Depending on the experimental conditions (gas pressure, wire bias voltage), the sputtering process occurs in different regimes corresponding to either the presence of a simple cathodic sheath or the breakdown of an hollow cathode type discharge inside the helix. Since the flux of metal atoms condensing onto the substrate depends on the sputtering characteristics, it is of particular interest to study the formation of this secondary discharge (breakdown voltage and density profile). This is carried out by measuring the Langmuir saturation current profile inside and around the helix, and correlating the results with the determination by Rutherford backscattering spectroscopy analysis of the deposition rate on a SiO₂ substrate. We also present density profiles of the argon plasma along the main axis of the reactor, which show that the flux of argon ions onto the substrate is not affected by the nature of the helix discharge. Finally, the ability to extend the range of deposition conditions (especially the ion flux to metal atom flux ratio) by using the helix diameter as an additional parameter is discussed.