In situ studies on the cohesive properties of α- and β-Ta layers on polyimide substrates

Despite much technological interest, a standardized test method for the quantification of the cohesive properties of thin films has not yet been established. In addition, for thin brittle coatings on compliant substrates, which are of great interest for flexible electronic devices, conventional techniques cannot be applied. In this work we use a synchrotron-based technique to determine biaxial stress-strain curves of Ta coatings on polyimide substrates in a uniaxial tensile test geometry and apply a two-dimensional shear lag model to derive the fracture strength and the fracture toughness of the coatings. The combined approach of fragment size analysis by in situ microscopy and stress measurement is used to establish a correlation between the modeled and the measured coating stress, which directly illustrates the range of applicability for our model. This technique is used to elucidate the influence of the crystallographic phase of Ta on its cohesive properties. The fracture strength and fracture toughness are greater for body-centered cubic α-Ta compared to tetragonal β-Ta, and plasticity may play a role in reducing stress concentrations at crack tips in α-Ta films. In addition, this study highlights the roles of film thickness, defect density and residual stress in the ability of brittle coatings to withstand considerably high strains without cracking.