Controlling the B/Ti ratio of TiBx thin films grown by high-power impulse magnetron sputtering

Abstract

TiBx thin films grown from compound TiB2 targets by magnetron sputter deposition are typically highly over-stoichiometric, with x ranging from 3.5 to 2.4, due to differences in Ti and B preferential-ejection angles and gas-phase scattering during transport from the target to the substrate. Here, the authors demonstrate that stoichiometric TiB2 films can be obtained using high-power impulse magnetron sputtering (HiPIMS) operated in power-controlled mode. The B/Ti ratio x of films sputter-deposited in Ar is controllably varied from 2.08 to 1.83 by adjusting the length of HiPIMS pulses ton between 100 and 30 μs, while maintaining average power and pulse frequency constant. This results in peak current densities JT,peak ranging from 0.27 to 0.88 A/cm2. Energy- and time-resolved mass spectrometry analyses of the ion fluxes incident at the substrate position show that the density of metal ions increases with decreasing ton due to a dramatic increase in JT,peak resulting in the strong gas rarefaction. With ton < 60 μs (JT,peak > 0.4 A/cm2), film growth is increasingly controlled by ions incident at the substrate, rather than neutrals, as a result of the higher plasma dencity and, hence, electron-impact ionization probablity. Thus, since sputter-ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the Ti concentration in as-deposited films increases with decreasing ton (increasing JT,peak) as ionized sputtered species are steered to the substrate by the plasma in order to maintain charge neutrality.