Ti atom and Ti ion number density evolution in standard and multi-pulse HiPIMS

In this paper, comparison of standard and multi-pulse high power impulse magnetron sputtering is performed. The effective branching fraction method is used for titanium atom and ion number density determination, showing that the residual titanium atoms and ions from the preceding pulse are crucial for the subsequent pulse initiation and development. It is shown that the discharge current rises faster in the subsequent pulse, but does not reach the same maximum as in the preceding pulse. The time evolution of the titanium atom density shows different behaviour, initial increase is followed by decrease in the preceding pulse and a rather constant evolution during the subsequent pulse. As for the titanium ion number density, it reaches typically lower values in the subsequent pulse, approaching the maximum values from the preceding pulse only at long delays of 1.5 ms. The most significant increase of the total ion flux to the substrate, namely 43% increase with respect to standard high power impulse magnetron sputtering, is observed in the multi-pulse high power impulse magnetron sputtering with the shortest studied delay of 200 mu s. The residual titanium atoms produced by the preceding pulse are already thermalized at the beginning of the subsequent pulse, thus being available for ionization during the subsequent pulse. The reservoir of these thermalized atoms gets depleted as the delay increases. However, even for the longest studied delay of 1.5 ms the influence of the preceding pulse on the subsequent pulse is still distinct, including the enhancement of the total ion flux to the substrate by 23%.