The characteristic shape of emission profiles of plasma spokes in HiPIMS: the role of secondary electrons

A time-resolved analysis of the emission of high power impulse magnetron sputtering (HiPIMS) plasmas reveals inhomogeneities in the form of rotating spokes. The shape of these spokes is very characteristic depending on the target material. The localized enhanced light emission has been correlated with the ion production. Based on these data, the peculiar shape of the emission profiles can be explained by the localized generation of secondary electrons, resulting in an enhanced outward diffusion. This general picture is able to explain the observed emission profile for different target materials including gas rarefaction and second ionization potential of the sputtered elements.