Formation and sustainment of spokes in planar dc magnetrons

Planar dc magnetrons with a conducting target are often used as an essential part of the film

deposition technique employing the PVD process. To reduce the interaction of neutrals sputtered

from the target with the working gas, the latter is typically kept at a low pressure, less than a few Pa.

To sustain the discharge at such a low pressure, a magnetic field confining electrons is required.

To increase the deposition rate, it is required to increase the power absorbed by the discharge,

which in turn demands an enhanced current. Experimental data suggest that the current increase

is often associated with observations of the rotating spoke structures. By utilizing 2D and 3D simulations

conducted with an implicit electrostatic energy-conserving PIC/MCC code, in the present work we argue that

formation and sustainment of spokes in planar dc magnetrons featuring an inhomogeneous magnetic field

results from a self-consistent interaction between the ionization and the potential landscapes,

which involves instabilities. Different mechanisms leading to the electron energization and the

related shaping of the ionization profile are addressed.