External magnetic field increases both plasma generation and deposition rate in HiPIMS

A widely recognised limitation of high power impulse magnetron sputtering (HiPIMS) is the lower deposition rate compared to that achieved using conventional DC sputtering. The HiPIMS deposition rate can be significantly increased by the application of an external magnetic field created by a solenoidal coil excited with a DC current pulse. However, the mechanisms causing enhancement of deposition rate are not fully understood. Here we investigate experimentally the influence of external magnetic fields on the sputtering conditions near the target and on the ion transport to the substrate, using an aluminium (Al) target as an example. The deposition rate was measured as a function of coil current and substrate bias voltage. We show that there is a favourable orientation of the applied field that increases the peak target current, substrate ion current, and deposition rate and an unfavourable one that inhibits the operation of the magnetron gun. To shed light on these observations and identify the mechanisms, we calculated magnetic field distributions using finite element methods. We show that a synergistic combination of the externally applied magnetic field and the inherent magnetron field expands and intensifies the ionisation zone. This increases the extent and density of the plasma leading to an increase in the deposition rate. We provide evidence that the external magnetic field also guides the ion flux towards the substrate and focuses it on the substrate, further increasing the deposition rate.