Pulsed-bias magnetron sputtering of non-conductive crystalline chromia films at low substrate temperature

Chromia coatings were produced by biased pulsed-dc sputter deposition in a dual-frequency (2F) mode, pulsing the target and substrate at frequencies of 130 kHz and 250 kHz, respectively. Crystalline α-Cr₂O₃ coatings were deposited at substrate temperatures as low as 90 °C, significantly less than the values reported in the literature (∼250 °C and more), exhibiting the potential to coat temperature sensitive substrates, such as polymers, with crystalline oxide films. We found that generating optimal ion bombardment conditions at the growing film surface is a critical factor in defining the structure of Cr₂O₃ coatings. Too low or too high energy ion bombardment can result in amorphous coatings, while a narrow window of optimal ion energies exists within which strongly crystalline coatings can be deposited at very low substrate temperatures. Also, we found that the conventional trend of decreasing deposition rate with increasing substrate bias voltage can be reversed when operating in 2F-pulsed-dc configuration, providing a combined benefit of both energetic ion-assisted deposition and a high deposition rate. The Cr₂O₃ films produced were found to possess hardness values of 23-27 GPa, approaching that of bulk Cr ₂O₃ and remaining approximately constant over the range of deposition parameters (and resultant coating structures) investigated. Results show that the 2F-pulsed-dc mode enhances the deposition process, in turn allowing an enhanced control of film structure and texture. Significantly less enhancement of the deposition process (and a reduced capability to manipulate beneficially the coating structure) is obtained when operating in one frequency (1F) synchronous pulsed-dc mode. The relative efficacy of the 2F-pulsed-dc processing configuration we believe to be due to additional plasma enhancement and to the fact that, in contrast to 1F-synchronous pulsed-dc configuration, the whole range of charged species (positive ions, negative ions, electrons, etc) abundant in such discharges can be deployed to beneficially modify coating growth conditions at the surface of the film. 2F-pulsed-dc sputter deposition therefore offers a straightforward, reproducible and easily scalable deposition process which, in addition to significant process improvements, also allows complex substrate heating and/or RF biasing systems to be avoided. The findings of this study are expected to be of general validity and applicability to certain other oxide (such as Al₂O₃, TiO₂ and HfO₂) and non-oxide coatings of scientific and commercial interest.