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 helicoid al wire which is negatively biased so that argon ions created in a high fre quency 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 eith er the presence of a simple cathodic sheath or the breakdown of an hollow c athode type discharge inside the helix. Since the flux of metal atoms conde nsing onto the substrate depends on the sputtering characteristics, it is o f particular interest to study the formation of this secondary discharge (b reakdown voltage and density profile). This is carried out by measuring the Langmuir saturation current profile inside and around the helix, and corre lating the results with the determination by Rutherford backscattering spec troscopy analysis of thr deposition rate on a SiO2 substrate. We also prese nt 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 ra nge of deposition conditions (especially the ion flux to metal atom flux ra tio) by using the helix diameter as an additional parameter is discussed.