Plasma–surface interaction at sharp edges and corners during ion-assisted physical vapor deposition. Part II: Enhancement of the edge-related effects at sharp corners

In the companion article, E. B. Macak et al., J. Appl. Phys. (2003) (Part I), we demonstrated that changes in the plasma sheath surrounding a sharp edge during ion-assisted physical vapor deposition (PVD) can lead to significant changes in the coating properties in the edge region. In this article, we examine the ion–surface interaction in the case of sharp corners. The three-dimensional corner geometry is shown—both experimentally and theoretically—to cause significant enhancement of the edge-related effects due to substantial increase in the ion flux density close to the corner. For TiAlN/VN coatings deposited on sharp corners of various geometries by closed-field unbalanced magnetron (CF UBM) sputtering using high flux low-energy Ar+-ion irradiation (Ji/Jme∼4, Ei=75−150 eV), the coatings in the corner region deteriorate rapidly with increasing the substrate bias during the deposition. At −75 V, delamination of the coatings, and at −105 V, partial resputtering can be observed. At −150 V, the coatings are completely resputtered from the corner area and the ion bombardment causes a significant erosion of the substrate creating complex topographical structures such as ripples and cones. Significant Al depletion (up to ∼70%) is found at the corners. The spatial extent of the morphological and compositional changes is also greatly enhanced (up to 3 mm from the edge in our conditions). We model the difference between the two-dimensional plasma sheath around an edge and the three-dimensional plasma sheath around a corner. The characteristic shape of the different plasma sheaths is used to calculate the corresponding magnitude of the ion bombardment and the spatial extent of the affected region. In the case of a 30° wedge coated by CF UBM at −150 V, the sputtering rate of the ions in the corner region is found to be up to ∼40 times higher than on flat parts—which explains the observed severe changes.