Measurement of the noibium/sapphire interface toughness via delamination

Niobium films were deposited on sapphire substrates using both physical vapor deposition (PVD) and ion beam assisted deposition (IBAD) at an ion energy of 1000 eV and an ion-to-atom arrival rate ratio of 0.4. The interface between the niobium film and the sapphire substrate was doped with up to 7.6 monolayers of silver. The films were patterned into fine lines using photolithography. During the photolithography process, curling and buckling were observed. The curling indicated a stress gradient in which the top of the film is tensile with respect to the bottom of the film, while buckling demonstrated that a portion of the film thickness must have been in compression. An analysis of the delamination showed that the critical energy release rate for the interface was on the order of 1 J m−2, and that the compressive stress is of the order 1 GPa. The higher energy release rate of the IBAD samples confirmed that the stronger interface is due either to the orientation relationship between the ion beam textured niobium film and the (0001) sapphire surface or the interface mixing caused by ion bombardment.