Micro-mechanical characterization of tantalum nitride thin films on sapphire substrates

The adhesion of Ta₂N thin films - often used as thin film resistors - to sapphire substrates has been studied by continuous microindentation and microscratch techniques. Ta₂N films, 0.10.63 μm in thickness, were sputter deposited onto single crystal substrates. Continuous microscratch experiments were performed by driving a conical diamond indenter simultaneously into and across the film surface until stresses high enough to delaminate the film were developed. Continuous microindentation experiments were performed to induce film spallation by normal indentation. From both of these experiments, interfacial fracture toughness was determined as a function of film thickness. The interfacial fracture toughness obtained from continuous microscratch experiments is 0.53±0.17 MPa√m, independent of film thickness. This observation indicates that there is almost no plastic deformation in the film prior to fracture so that a `true' interfacial fracture toughness is measured. For the 0.63 μm thick film, continuous microindentation data yielded a fracture toughness of 0.61±0.08 MPa√m, which matches closely the value obtained from the microscratch test. Hence, the continuous microscratch and microindentation techniques are viable methods for determining the interfacial fracture toughness in such bi-material systems.