How microalloying of the Al target can improve process and film characteristics of sputtered alumina

The outstanding thermo-mechanical and chemical stability of Al2O3 thin films attracts particular attention in academia and industry. Here we show that alloying of the powder metallurgically prepared Al targets with 2 as well as 5 at.% of Cr, Mo, or W significantly improves the process stability (e.g., reducing arcing events) for Al2O3, allowing their reactive magnetron sputtering in DC mode (substrate temperature was always 360 °C). Contrary to these microalloying elements, Nb did not change or improve the process characteristics of the Al target due to the relatively coarse Nb particles (<125 μm). Particularly the small (<10 μm) and fine-dispersed W particles are very effective in stopping the collision cascades to concentrate them to the near subsurface target-regions. This leads to a shift of the target-poisoning onset from O2/(Ar + O2) flow-rate-ratios of about 32 to 52%, specifically when adding 5 at.% to the Al target. Thereby also improved deposition rates are possible. However, detailed X-ray diffraction (XRD) and transmission electron microscopy studies show that only thin films developed from the 2 at.% W alloyed Al target have a nanocrystalline γ-Al2O3-based structure comparable to those prepared from Al or Cr alloyed Al targets. The films are with 28.3 GPa instead of 26.8 GPa slightly harder when sputtered from Al0.98W0.02 instead of Al targets. Higher W contents (and also Nb) in the target lead to the formation of Al2O3-based films with considerably lower crystalline phase fractions and hardness (~13 GPa). Hardest films, with 30.0 and 29.6 GPa, are obtained when using Al0.98Cr0.02 and Al0.95Cr0.05 targets, respectively. The formation of volatile Mo-oxides during film growth interferes the structure development, leading to rather soft films with 8.0 GPa especially when using the higher Mo alloyed Al0.95Mo0.05 target. Based on our results we can conclude that microalloying the Al targets with small and fine-dispersed Cr or W particles not just improves process stability and deposition rate during reactive sputtering of Al2O3, but also their mechanical thin film properties.