Reactive HiPIMS deposition of Al-oxide thin films using W-alloyed Al targets

( )The outstanding oxidation resistance, thermo-mechanical stability, and chemical inertness of alumina, but also the synthesis of phase pure polymorphs attract particular attention in academia and industry. Especially, the difficulties regarding the synthesis of alpha- or gamma-structured Al2O3 by physical vapor deposition techniques are still strong limitations. Within this study, we investigated in detail the influence of 2 at.% tungsten in the Al-target on the process stability and phase formation during reactive DC magnetron sputtering as well as high power impulse magnetron sputtering (HiPIMS) of Al2O3-based coatings. The small addition of W to the Al target allows to increase the oxygen partial pressure by more than 200% while maintaining a stable deposition process. Ion mass spectroscopy measurements yield a promising high fraction of O-16(+) and O-32(4)+, when operating the W-containing target in the metal-to-poisoned transition mode. A significant increase of O-16(+) is further provided by the target surface oxide in poisoned mode. Detailed time-of-flight ion mass spectroscopy investigations during one HiPIMS pulse show a clear temporal separation of the individual ions arriving at the substrate plane during the pulse on-time, allowing for controlled ion attraction by synchronizing the bias pulse to the discharge impulse. Equal amounts of Al-27(+) and O-32(2)+ can be attracted using a bias on-time between 400 mu s and 900 mu s is in the off-time (after glow) leading to a dense and nano-crystalline coating. Detailed electron microscopy investigations show the presence of metallic phase fractions for higher duty cycles (7.5%). Decreasing the duty cycle to 3.75% leads to amorphous coatings when operating the Al-target at the highest oxygen partial pressure in metallic mode.

Automated summary

This study focuses on the synthesis and stability of Al2O3 coatings. By adding a small amount of tungsten to the Al target, the oxygen partial pressure can be increased while maintaining a stable deposition process, resulting in dense and nano-crystalline coatings. Controlled ion attraction can be achieved by synchronizing the bias pulse to the discharge impulse under specific conditions.