The deposition of aluminium oxide coatings by reactive unbalanced magnetron sputtering

The problems associated with the reactive d.c. sputtering of highly insulating materials, such as alumina, are well documented. Deposition rates are low and an insulating layer can build up on the surface of the target, causing arcing. Arc events prevent stable operation and can result in droplets of material being ejected from the target. Such droplets can cause defects in the growing film. However, studies have shown that the formation of arcs can be significantly reduced if the magnetron discharge is pulsed at a frequency in the 10-200 kHz range. In this investigation, AlO_x (where 0.7≤x≤1.5) coatings were deposited by reactive unbalanced magnetron sputtering using either a d.c. power supply in series with a fixed 20 kHz pulse unit, or a variable frequency supply with a maximum frequency of 33 kHz (for comparison purposes, coatings were also deposited by reactive d.c. sputtering, without pulsing the discharge). Deposition parameters were varied systematically to produce a range of coating compositions and properties. The resulting coatings ranged from extremely dense, stoichiometric Al₂O₃ films, with Knoop microhardness values > 2500 kg mm^-2, to very soft (< 100 kg mm^-2) columnar, sub-stoichiometric films. Deposition rates varied from 4 to 20 μm h^-1. Some initial results of wear tests carried out on these coatings are also reported. The pulsed power supplies were found to be very stable in operation, with very few arc events being observed.