For optimal electric performance, a flexible electronic device has to sustain large mechanical stresses without losing its structural integrity. The thin film geometry exerts hard constraints or even excludes a number of conventional techniques for mechanical testing. For the determination of yield strength, synchrotron x-ray diffraction (sXRD) is commonly applied. However, it is restricted by beam time availability and new labscale characterization techniques are desirable.
In this study we present and compare two different methods (sXRD and Reflection Anisotropy Spectroscopy (RAS)) for mechanical characterization of Cu thin films on polyimide substrates. Both techniques are known to be sensitive to changes in the strain state of the specimen, its phase and microstructural configurations. Since the mechanical behavior of thin metallic films is known to be thickness dependent specimens of thicknesses ranging from 50 to 500 nm were investigated.
In an earlier work Wyss et al. showed that the evolution of the RA-signal at a specific energy is proportional to the elastic strain in the material. However a full correlation and yield point determination has not been demonstrated yet.
In classical stress-strain curves, obtained for example by sXRD, the yield point is usually determined in the loading regieme by shifting the linear slope (in the elastic regieme) by 0.2% and determining the intersection with the stress strain curve (Rp0.2). The startpoint has to be shifted according to the residual stresses measured for example with sin2(ψ).
Since this yields a lot of problems, unloading curves are used for yield point determination since they are independent of the initial residual stresses and errors caused by the mounting procedure of the specimen.
Our results show that RAS, compared to sXRD, is a suitable and reliable method that allows dynamic monitoring of thin film strain states during deformation and can be used for yield point determination of thin metallic films.