Time-resolved investigation of an asymmetric bipolar pulsed magnetron discharge: Influence of pressure

A bipolar pulsed magnetron deposition discharge has been studied with pulse frequencies of 100 and 150 kHz, respectively. The discharge was operated in an argon/oxygen mixture at different total pressures with a circular magnesium target as cathode. Time-resolved Langmuir double probe measurements and time-resolved optical emission spectroscopy have been used to analyse the temporal behaviour of the pulsed discharge in the substrate region. It is found that typical structures in the time dependence of both charge carrier density and optical emission which are observed in the “on”-phase of the discharge are significantly influenced by the pressure. If the pressure is increased, the discharge is built up faster with all the structures appearing earlier. Additionally, the intensity of the structures is changed. We demonstrate that this on the one hand due to enhanced collisions of the electrons with neutral gas particles. On the other hand, the wave form of the target voltage is altered in connection with the increased pressure which subsequently feeds back to the discharge. A study of the temporal behaviour of the argon emission in the “off”-phase reveals an approximately exponential decay which is also faster when the pressure is increased. By comparison of argon lines which are either predominantly excited by direct transition or via cascades from upper levels we found that at higher pressure cascading plays a less important role as the density of high energy electrons is reduced.