SiN_x Coatings Deposited by Reactive High Power Impulse Magnetron Sputtering: Process Parameters Influencing the Nitrogen Content

Abstract

Reactive high power impulse magnetron sputtering (rHiPIMS) was used to deposit silicon nitride (SiNx) coatings for bio-medical applications. The SiNx growth and plasma characterization were conducted in an industrial coater, using Si targets and N2 as reactive gas. The effects of different N2-toAr flow ratios between 0 and 0.3, pulse frequencies, target power settings and substrate temperatures on the discharge and the N content of SiNx coatings were investigated. Plasma ion mass spectrometry shows high amounts of ionized isotopes during the initial part of the pulse for discharges with low N2-to-Ar flow ratios of < 0.16, while signals from ionized molecules rise with the N2-to-Ar flow ratio at the pulse end and during pulse-off times. Langmuir probe measurements show electron temperatures between 2 -3 eV for non-reactive discharges and 5.0 to 6.6 eV for discharges in transition mode. The SiNx coatings were characterized with respect to their composition, chemical bond structure, density and mechanical properties by X-ray photoelectron spectroscopy, X-ray reflectivity, X-ray diffraction, and nanoindentation, respectively. The SiNx deposition processes and coating properties are mainly influenced by the N2-to-Ar flow ratio and thus by the N content in the SiNx films and to a lower extent by the HiPIMS frequencies and power settings as well as substrate temperatures. Increasing N2-to-Ar flow ratios lead to decreasing growth rates, while the N contents, coating densities, residual stresses and the hardnesses increase. These experimental findings were corroborated by density functional theory calculations of precursor species present during rHiPIMS.

SiNx coatings deposited by reactive high power impulse magnetron sputtering: Process parameters influencing the residual coating stress

“…44 In our previous study we found that the SiN x coatings were amorphous 12 hence, the latter applies for the here investigated coatings.…”

“…6(a)-6(d) for different power and frequency settings. Although, ion energies are generally lower in processes using f N 2 =Ar of 0.28 due to the temporarily poisoned target and N 2 dissociation, ionization as well as excitation, 12 the results in Figs. 6(a)-6(d) suggest higher energies and amounts of ions as the average target power (cf.…”

“…Global deposition parameter influencing the residual coating stress Our previous study 12 showed that increasing N-contents up to $51 at. % in the coating, result in increasing compressive stresses up to À1.5 GPa.…”

“…The presence of Si-Si bonds, in turn, promotes a lowered density and hardness of the coatings. 12,[44][45][46] For SiN x coatings with N-contents of !50 at. %, effects induced by an increased substrate temperature or forward sputtering on the tetrahedral bond coordination of Si, and thus also on the coating properties, could not be revealed but can also not be excluded, due to the amorphous nature of the coatings.…”

“…In an earlier study, 12 we demonstrated the growth of amorphous SiN x coatings and presented the activation paths for the N 2 molecule during reactive high power impulse magnetron sputtering (rHiPIMS) at different N 2 /Ar flow (f N 2 =Ar Þ ratios. Here, the use of f N 2 =Ar between 0.28 and 0.3 yield SiN x coatings with approximately 50 at.…”