Studies of mid-frequency pulsed dc biasing

The application of mid-frequency (100–350 kHz) pulsed dc power at the substrate is a recent development in the magnetron sputtering field. 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. For example, under a particular set of operating conditions, a threefold increase in ion current was observed at a bias voltage of –300 V when the bias was pulsed at 350 kHz, compared to the dc case. This phenomenon is believed to be due to the initiation of a second discharge at the substrate. 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. Consequently, the variation in ion current with pulse frequency and bias voltage has been studied for an unbalanced magnetron sputtering system. In addition, substrate heating rates, current–voltage wave forms and plasma characteristics have also been investigated. A series of TiO2 and TiN films were then grown under different bias conditions. Analysis of these films showed that the application of pulsed dc power at the substrate can significantly influence film structure and properties. In particular, shifts in crystalline structure and texture were observed