Residual Stress Characterisation of Thin Sputtered Copper Films on Silicon Exploiting Membrane Resonance within a Specimen Centred Approach

Thin metal layers, especially those made of copper, are omnipresent in today’s packaging applications as e.g. RDL structures, conductor traces on flexible and stretchable substrates, chip finishes or terminal metallisation, serving electrical, thermal or mechanical purposes. As Cu is very process and size dependent, especially thin layers will display residual stresses, textures, inhomogeneity or damage, all of which determine its properties as e.g. elastic and (visco-)plastic behaviour, fatigue resistance etc. As thin layers are extremely difficult to handle, a characterisation method to examine thermo-mechanical properties of thin metal films within a specimen centred approach (no transfer, no clamping of samples, customised testing equipment) is proposed based on dynamic thin film membrane excitation and resonance spectrum analysis by a simple, fast and low cost method involving electrodynamic excitation. The method produces very good accuracy when benchmarked against scanning laser Doppler vibrometry.