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

摘要

The residual coating stress and its control is of key importance for the performance and reliability of silicon nitride (SiN_x) coatings for biomedical applications. This study explores the most important deposition process parameters to tailor the residual coating stress and hence improve the adhesion of SiN_x coatings deposited by reactive high power impulse magnetron sputtering (rHiPIMS). Reactive sputter deposition and plasma characterization were conducted in an industriai deposition chamber equipped with pure Si targets in N_2/Ar ambient. Reactive HiPIMS processes using N_2-to-Ar flow ratios of 0 and 0.28-0.3 were studied with time averaged positive ion mass spectrometry. The coatings were deposited to thicknesses of 2 µm on Si(001) and to 5 µm on polished CoCrMo disks. The residual stress of the X-ray amorphous coatings was determined from the curvature of the Si substrates as obtained by X-ray diffraction. The coatings were further characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and nanoinden-tation in order to study their elemental composition, morphology, and hardness, respectively. The adhesion of the 5 µm thick coatings deposited on CoCrMo disks was assessed using the Rockwell C test. The deposition of SiN_x coatings by rHiPIMS using N_2-to-Ar flow ratios of 0.28 yield dense and hard SiN_x coatings with Si/N ratios <1. The compressive residual stress of up to 2.1 GPa can be reduced to 0.2 GPa using a comparatively high deposition pressure of 600 mPa, substrate temperatures below 200 °C, low pulse energies of <2.5 Ws, and moderate negative bias voltages of up to 100 V. These process parameters resulted in excellent coating adhesion (ISO 0, HF1) and a low surface roughness of 14 nm for coatings deposited on CoCrMo,