Thermo-mechanical characterisation of thin sputtered copper films on silicon: Towards elasto-plastic, fatigue and subcritical fracture-mechanical data

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. During operation, thermo-mechanical stress will cause failures in the Cu layers and interfaces over time. As Cu is very process and size dependent, its resistance to fatigue failure needs to be characterised with samples which have undergone identical processing steps as those in the real application. For that purpose, simple specimens and fast testing routines are necessary, some of which may need special loading stages for varying the load variables of interest such as stress amplitude and temperature. This paper addresses fatigue characterisation of thin Cu films on silicon under typical processing conditions on simple and inexpensive but industry-grade samples. Along with them, custom built test stands have been used to handle those specimens appropriately within a specimen-centred approach.