Nanoindentation as part of material characterization of thin metal films

Thin metal films have been present in microelectronic packaging for a long time as e.g. RDL and BEOL structures, terminal metal or shielding layers. In most use cases, thin metal layers will be thermo-mechanically loaded, leading e.g. to disruption due to high thermal transients, or to fatigue cracks due to periodic sub-critical bending, thus causing reliability problems. In order to understand, describe and model the underlying failure mechanisms within a physics-of-failure approach, it is important to characterize thin metal films for their thermo-mechanical properties. It is known, that thin metals are highly process dependent in their properties. Also, the small thickness introduces size effects, so methods requiring thick or bulk samples are not of much use. This paper discusses the applicability and challanges of nanoindentation (NI) on thin metal films. To that end, nanoindentation and atomic force microscopy (AFM) measurements on Ni, Cu, and Al are investigated together with finite element simulations to obtain elastic-plastic material properties.