Gas rarefaction effects during high power pulsed magnetron sputtering of groups IVb and VIb transition metals in Ar

The authors use energy- and time-dependent mass spectrometry to analyze the evolution of metal- and gas-ion fluxes incident at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of groups IVb and VIb transition-metal (TM) targets in Ar. For all TMs, the time-and energy-integrated metal/gas-ion ratio at the substrate plane NMe+/NAr+ increases with increasing peak target current density JT,peak due to rarefaction. In addition, NMe+/NAr+ exhibits a strong dependence on metal/gas-atom mass ratio mMe/mg and varies from ∼1 for Ti (mTi/mAr = 1.20) to ∼100 for W (mW/mAr = 4.60), with JT,peak maintained constant at 1 A/cm2. Time-resolved ion-energy distribution functions confirm that the degree of rarefaction scales with mMe/mg: for heavier TMs, the original sputtered-atom Sigmund-Thompson energy distributions are preserved long after the HiPIMS pulse, which is in distinct contrast to lighter metals for which the energy distributions collapse into a narrow thermalized peak. Hence, precise timing of synchronous substrate-bias pulses, applied in order to reduce film stress while increasing densification, is critical for metal/gas combinations with mMe/mg near unity, while with mMe/mg≫1, the width of the synchronous bias pulse is essentially controlled by the metal-ion time of flight. The good agreement between results obtained in an industrial system employing 440 cm2 cathodes and a laboratory-scale system with a 20 cm2 target is indicative of the fundamental nature of the phenomena.