Magnetron sputtering: Illuminating physics of ionization zones

Magnetron sputtering is commonly used vapor deposition technique for the preparation of thin films and coatings. Until recently it was believed that magnetron plasma is homogeneously distributed in a ring-shaped region above the cathode. Investigations by ICCD cameras and other time-resolved techniques changed this view. Namely, plasma is concentrated in dense regions that are called "ionization zones" or "spokes". Ionization zones were first observed in pulsed discharges (i.e., HiPIMS) [1] and later in continuously run discharges (i.e., DCMS) [2]. They are usually organized in semi-periodic patterns and exhibit an arrowhead-like shape. In general, ionization zones in DCMS have a longer azimuthal length, whereas in HiPIMS they are more numerous and azimuthally shorter. Dynamics of zones strongly depends on the discharge conditions. In a low-current DCMS discharges zones move in the -E×B direction, while in a high-current DCMS or HiPIMS discharges they move in the E×B direction [3].In the talk we will present our understanding of the ionization zone phenomenon. The formation, sustainability, organization and dynamics of ionization zones will be reviewed [4]. Measurements of the plasma potential by emissive probe show highly non-uniform potential distribution with strong electric fields at the edge of the ionization zone. Such fields strongly affect motion and energy of charged particles. A self-sustaining feedback loop exists between the potential structure, electron heating and ionization processes as electrons drift in the magnetic trap of the magnetron. We suggest that a moving double layer plays a crucial role in the energization of electrons and is in large part responsible for sustaining the discharge [5]. (References: [1] A. Anders et al., J. Appl. Phys. 111 (2012) 053304; [2] M. Panjan et al., Plasma Sources Sci. Technol. 24 (2015) 065010; [3] Y. Yang et al., Appl. Phys. Lett. 105 (2014) 254101; [4] M. Panjan et al., Plasma Sources Sci. Technol. 23 (2014) 025007; [5] M. Panjan and A. Anders, J. Appl. Phys. accepted for publication)