Quantification and control of the fraction of ionization of the sputtered species are crucial in magnetron sputtering, and in particular in high-power impulse magnetron sputtering (HiPIMS), yet proper definitions of the various concepts of ionization are still lacking. In this contribution, we distinguish between three approaches to describe the degree (or fraction) of ionization: the ionized flux fraction Fflux, the ionized density fraction Fdensity, and the fraction α of the sputtered metal atoms that become ionized in the plasma (sometimes referred to as probability of ionization). By studying a reference HiPIMS discharge with a Ti target, we show how to extract absolute values of these three parameters and how they vary with peak discharge current. Using a simple model, we also identify the physical mechanisms that determine Fflux, Fdensity, and α as well as how these three concepts of ionization are related. This analysis finally explains why a high ionization probability does not necessarily lead to an equally high ionized flux fraction or ionized density fraction.
Bibliographical noteFunding Information:
JTG and NB gratefully acknowledge the hospitality of CNRS and University Paris-Sud, Orsay, where much of this study was conducted. NB also acknowledges the support of Svensk-Franska Stiftelsen. The authors acknowledge Dr. T. J. Petty at University Paris-Sud, Orsay, for his work on figure 1. The work was partially supported by the Icelandic Research Fund Grant No. 130029 and the Swedish Government Agency for Innovation Systems (VINNOVA) contract no. 2014-04876.
© 2018 IOP Publishing Ltd.
- high power impulse magnetron sputtering (HiPIMS)
- magnetron sputtering