Recent developments in pulsed magnetron sputtering

In magnetron sputtering, coating properties are largely determined by the rate at which energy is delivered to the growing film through simultaneous ion bombardment. The structure and properties depend on three parameters: the homologous temperature; the ratio of the fluxes of bombarding ions and depositing atoms; and the energy of the bombarding ions. It is shown here that pulsing the target voltage can strongly influence these intrinsic parameters and can cause much greater energy fluxes to be delivered to the substrate than in DC systems operating at the same input power. Time averaged Langmuir probe measurements have shown that electron temperatures, ion and electron densities and ion fluxes to the substrate all increase with increasing pulse frequency, and that a burst of "hot" electrons is associated with the fast negative voltage transient between "reverse" and "on" phases of the target voltage. It is shown that the energies of the ions arriving at the substrate are influenced by both the reverse time/duty factor and the pulse frequency, and that that these energy variations have very significant effects on coating structure and properties. The effects of applying mid-frequency (100-350 kHz) pulsed dc power at the substrate have also been studied. It has been found that, unlike the dc case, if the bias is pulsed in this range, the current drawn at the substrate does not saturate, but continues to increase with increasing bias voltage. In addition, this effect becomes more marked as the pulse frequency is increased. Pulsing the substrate bias voltage, therefore, offers a novel means of controlling the ion current drawn at the substrate. Clearly, this has significant implications in relation to film growth, sputter cleaning, and substrate preheating processes. (C) 2004 Elsevier B.V. All rights reserved.