Enhanced deposition rates in plasma sputter deposition

Langmuir probe and emission spectroscopic measurements are performed in a high frequency (100 MHz) argon plasma used for the sputter deposition process of thin films of palladium (dedicated to catalysis applications). The metal source is a helicoidal palladium wire which is negatively biased with respect to the plasma potential. This induces sputtering by the ions present in the plasma. The probe results show that the presence of the helicoidal wire in the chamber does not affect the total ion flux at the substrate location. However, as the bias voltage on the wire and/or the argon pressure are increased, a secondary direct current (DC) discharge is created inside the helicoidal wire which follows a Paschen-like law; the breakdown voltage is lower than in the case of a conventional Ar discharge, probably as a result of the presence of primary electrons generated by the main high frequency (HF) plasma. This second discharge is characterized by a strong Ar(+) flux peak inside the helicoidal wire, which probably arises from a hollow cathode type discharge. From emission spectroscopy and deposition analysis, it is shown that this secondary plasma causes an increase of the sputtered Pd atom number and, consequently, an enhanced deposition rate.