Thickness dependence of the electro-mechanical response of sputter deposited Mo thin films on polyimide: Insights from in situ synchrotron diffraction tensile tests

The in situ characterization of the deformation and fracture behavior of brittle metal films is of great technological interest for many modern applications. A prominent example is the field of flexible electronics, which rely on the electrical and mechanical integrity of metal thin films on compliant substrates when exposed to straining or bending. Within this work, failure mechanisms, such as cracking and buckling, were studied as a function of film thickness and correlated with the elastic-plastic material response during straining. Mo thin films were synthesized with thicknesses between 40 and 500 nm on polyimide substrates using an industrial scale in-line direct current magnetron sputtering system. In situ synchrotron X-ray diffraction was employed to determine the evolution of lattice strain and film stress during uniaxial tensile straining while simultaneously measuring the change in electrical resistance. The results highlight that the electro-mechanical properties of Mo thin films scale with the film thickness. In general, a significant increase in fracture strength was observed with decreasing film thickness. Thus thinner Mo films were able to withstand higher tensile strains before cracking. In contrast, the 40 nm thick Mo film was the earliest to show delamination in the form of buckles, but the applied tensile strain of 12% was not high enough to induce buckling in the 500 nm thick Mo film. A model based on film buckling under uniaxial tensile tests was used to assess the interfacial adhesion energy between the Mo films and the polyimide substrates yielding values between 11 +/- 1 and 14 +/- 5 J/m(2).