Controlling the Composition of MeB_2±δ (Me = Al, Ti, Cr, W) Thin Films Grown by DC Magnetron Sputtering and HiPIMS

Controlling film composition is essential for engineering the properties of thin films. One system that has proven particularly sensitive to deposition process parameters is titanium boride, TiB_x; films grown by direct current magnetron sputtering (DCMS) from a TiB₂ target are usually over-stoichiometric (2.0 ≤ x ≤ 3.0), due to different angular distribution of the Ti and B deposition flux. On the other hand, films grown by high power impulse magnetron sputtering (HiPIMS), tend to be under-stoichiometric (1.4 ≤ x ≤ 2.0), due to the much higher degree of ionization, especially of Ti, which leads more Ti⁺ ions to be guided by the magnetic field toward the substrate, and hence reducing the B/Ti ratio. Other transition metal borides have been reported to show none, or only little, of this effect, for reasons not well understood.<br/>In this work, we deposit four different metal-diborides; AlB₂, TiB₂, CrB₂ and WB₂, by DCMS and HiPIMS, and compare compositions, measured by RBS, ERDS and EDS, of samples mounted at 0°, 30° and 60° relative to the surface normal of the 3-inch-diameter targets. This allows us to study the effect of the distinctly different atomic masses, and ionization potentials, on the angular distribution of the deposition flux. All films are grown in an Ar atmosphere at a pressure of 5 mTorr (0.67 Pa), with no substrate heating.<br/>The resulting film boron-to-metal ratios vary between ~1.3 and 3.0, depending on deposition condition and sample position, with different trends for the different elements, in a seemingly inconsistent way. Thus, in order to understand the experimental results, the sputtering and re-sputtering effects due to energetic particle bombardment at the target and substrate, respectively, were simulated by the SRIM, TRIDYN and IMSIL software. The calculated angular and energy distributions of the sputtered species from the target were then used to estimate the trajectories of ions through the magnetic field from the magnetron due to the Lorentz force. Even though the effects of plasma interactions, gas rarefaction, and the “sputtering wind” cannot easily be simulated, this does give insight into how the ion flux distribution will differ from the neutral flux, which, in turn, can explain differences between films grown by DCMS vs. HiPIMS.<br/>The combined experimental and simulated results provide us with an improved understanding of how the film composition depends on preferential emission angle, ionization potential, atomic mass, and gas phase transport of the different elements.