This work uses a novel approach of combined medium frequency (20 – 350 kHz) asymmetric bipolar pulsed magnetron sputtering (pulsed-DC) and pulsed substrate biasing to reactively deposit Cr2O3 and Al2O3 thin films at substrate temperatures of the order of 100 °C. Usually in such cases, the target and the substrate are pulsed at the same frequency [single-frequency (1F) mode] in either synchronous (master-slave) or asynchronous mode. The latter is however rarely used, as it can cause instabilities in the deposition process. Another possible mode of operation, which we exploit in this paper, is when target and substrate are pulsed at frequencies that are significantly different [dual-frequency (2F) mode] thus assuring the repeatability and stability of the process. In this paper we report results achieved using this novel sputter deposition approach for dielectric oxide coatings.
Cr2O3 and Al2O3 coatings were produced using the abovementioned 2F-pulsed-DC sputter deposition configuration, pulsing the target and substrate at frequencies of 130 and 250 kHz respectively. α-Cr2O3 coatings were produced at substrate temperatures as low as 90 °C. Al2O3 coatings containing some crystalline constituents (κ and/or θ phases) were obtained at a substrate temperature in a range of 124-158 °C. We found that generating optimal ion bombardment conditions at the growing film surface is a critical factor in defining the structure of Cr2O3 and Al2O3 coatings. Too low or too high energy ion bombardment results in amorphous coatings, while a narrow window of optimal ion energies exists within which crystalline coatings can be deposited at very low substrate temperatures. The effectiveness of the 2F-pulsed-DC processing configuration is due to the fact that, in contrast to the 1F-synchronised pulsed-DC configuration, it allows the whole range of charged species (positive ions, negative ions and electrons) abundant in such discharges to be deployed in beneficially modifying coating growth conditions at the surface of the film. Little or no enhancement of the deposition process (and resultant coating structure) is obtained when operating in 1F-synchronous pulsed-DC mode. The findings of this study are expected to be of general validity and applicability to other (oxide and non-oxide) coating systems.