Control of the metal/gas ion ratio incident at the substrate plane during high-power impulse magnetron sputtering of transition metals in Ar

Publication date: 30 November 2017
Source:Thin Solid Films, Volume 642
Author(s): G. Greczynski, I. Zhirkov, I. Petrov, J.E. Greene, J. Rosen
High-power impulse magnetron sputtering (HiPIMS) of materials systems with metal/gas-atom mass ratios m Me /m g near, or less than, unity presents a challenge for precise timing of synchronous substrate-bias pulses to select metal-ion irradiation of the film and, thus, reduce stress while increasing layer density during low-temperature growth. The problem stems from high gas-ion fluxes F g ⁺ (t) at the substrate, which overlap with metal-ion fluxes F Me ⁺ (t). We use energy- and time-dependent mass spectrometry to analyze F Me ⁺ (t) and F g ⁺ (t) for Group IVb transition-metal targets in Ar and show that the time-and energy-integrated metal/gas ion ratio N Me ⁺ /N Ar ⁺ at the substrate can be controlled over a wide range by adjusting the HiPIMS pulse length τ ON , while maintaining the peak target current density J T,peak constant. The effect is a consequence of severe gas rarefaction which scales with J T (t). For Ti-HiPIMS, terminating the discharge at the maximum J T (t), corresponding to τ ON =30μs, there is an essentially complete loss of Ar⁺ ion intensity, yielding N Ti ⁺ /N Ar ⁺ ~60. With increasing τ ON , J T (t) decreases and N Ti ⁺ /N Ar ⁺ gradually decays, due to Ar refill, to ~1 with τ ON =120μs. Time-resolved ion-energy distribution functions confirm that the degree of rarefaction depends on τ ON : for shorter pulses, τ ON <60μs, the original sputtered-atom Sigmund-Thompson energy distributions are preserved long after the HiPIMS pulse, which is in distinct contrast to longer pulses, τ ON ≥60μs, for which the energy distributions collapse into narrow thermalized peaks. Thus, optimizing the HiPIMS pulse width minimizes the gas-ion flux to the substrate independent of m Me /m g .