Understanding ion and atom fluxes during high-power impulse magnetron sputtering deposition of NbC_x films from a compound target

A combination of time-averaged mass spectroscopy (MS), time-averaged optical emission spectroscopy (OES), and plasma transport modeling was employed to understand the transport processes of ions and atoms in high-power impulse magnetron sputtering discharges resulting in changes in the stoichiometry of NbCx films during their deposition from a stoichiometric NbC compound target at different repetition frequencies and duty cycles. Mass spectrometry showed that the fluxes of ions originating from the elements of the target increase with increasing pulse power density (and decreasing pulse length) due to an increasing electron density and, thus, electron-impact ionization probability. Due to the higher ionization energy and much lower ionization cross section of C (compared to Nb), it was found that the contribution of C+ ions to the deposition flux is practically negligible. Additionally, OES tracked the densities of ions and atoms at different distances from the target. The OES analysis revealed that the atom densities decreased as the pulse power density increased. In contrast, the ion densities exhibited an increase, which is consistent with the findings of MS. Using the data from MS, OES, and modeling, we were able to estimate the fluxes of atoms to the substrate. Our observations demonstrated a transition from C-rich toward Nb-rich flux of film-forming species with increasing pulse power density, corresponding to changes in the film composition. We further discuss the role of internal plasma processes that are responsible for this transition.