Strategy for tuning the average charge state of metal ions incident at the growing film during HIPIMS deposition

Energy and time-dependent mass spectrometry is used to determine the relative number density of singly- and multiply-charged metal-ion fluxes incident at the substrate during high-power pulsed magnetron sputtering (HIPIMS) as a function of the average noble-gas ionization potential. Ti is selected as the sputtering target since the microstructure, phase composition, properties, and stress-state of Ti-based ceramic thin films grown by HIPIMS are known to be strongly dependent on the charge state of Tin+ (n = 1, 2, …) ions incident at the film growth surface. We find that the flux of Tin+ with n > 2 is insignificant; thus, we measure the Ti2+/Ti+ integrated flux ratio JTi2+ =JTi+ at the substrate position as a function of the choice of noble gase Ne, Ar, Kr, Xe, as well as Ne/Ar, Kr/Ar, and Xe/Ar mixtures – supporting the plasma. We demonstrate that by changing noble-gas mixtures, JTi2+ varies by more than two orders of magnitude with only a small change in JTi+ . This allows the ratio JTi2+ =JTi+ to be continuously tuned from less than 0.01 with Xe, which has a low first-ionization potential IP1, to 0.62 with Ne which has a high IP1. The value for Xe, IP1Xe= 12.16 eV, is larger than the first ionization potential of Ti, IP1Ti= 6.85 eV, but less than the second Ti ionization potential, IP2Ti= 13.62 eV. For Ne, however, IP1Ne= 21.63 eV is greater than both IP1Ti and IP2Ti. Therefore, the high-energy tail of the plasma-electron energy distribution can be systematically adjusted, allowing JTi2+/JTi+ to be controllably varied over a very wide range