Substrate effects during mid-frequency pulsed DC biasing

The use of pulsed DC power at the substrate is a recent development in the field of magnetron sputtering. Pulsing the substrate bias voltage in the mid-frequency range (100–350 kHz) has been found to significantly increase the ion current drawn at the substrate. For DC bias applications, it is normally found that the current drawn at the substrate saturates at bias voltages of the order of −100 V. Further increases in bias voltage do not lead to an associated increase in ion current. However, recent experiments have shown that if the bias voltage is pulsed, not only is the magnitude of the ion current greater than for the DC bias case, but this current also continues to increase as the bias voltage is increased. In addition, both of these effects become more marked as the pulse frequency is increased. For example, under a particular set of operating conditions, a three-fold increase was observed in the current drawn at the substrate as the bias voltage was increased from −100 to −300 V and the bias pulse frequency was increased from 0 to 350 kHz. 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 pre-heating processes. Consequently, the variations in ion current with pulse frequency and bias voltage, and the associated substrate heating effects, have been studied for an unbalanced magnetron sputtering system. The influence of these parameters on the properties of reactively sputtered titania films is also reported.