Formation of crystalline γ-Al2O3 induced by variable substrate biasing during reactive magnetron sputtering

Reactive magnetron sputtering is a widely used technique to deposit various materials such as oxides and nitrides with a superior control of morphology and stoichiometry. The adjustment of the film properties at a given substrate temperature is believed to be affected by the average energy 〈E〉 per incorporated atom during film growth, which is controlled by the ion-to-neutral ratio in the film forming growth flux and the energy of the incident ions. This concept is tested for alumina growth in an rf-magnetron discharge by keeping 〈E〉, the average energy of the incident ions Eions, and the ion-to-neutral flux ratio constant, but varying only the energy distribution of the incident ions (ion energy distribution–IED). The influence of the IED on film growth is monitored by observing the transition of the films between x-ray amorphous Al2O3 to γ-Al2O3. The results reveal that the substrate temperature necessary for the transition to γ-crystalline films can be lowered by almost 100 °C, when the maximum energy of the incident ions is kept at 100 eV, while maintaining the energy per incorporated atom at 11 eV. This result is compared with TRIM calculations for the collision cascades of impacting ions.