Mechanismen und Größeneffekte von Ermüdungsschädigungen in dünnen Kupfer filmen bei sehr hohen Zyklenzahlen

Abstract

Thin metal films show completely different fatigue behavior than bulk metals. The films have longer fatigue lives and instead of extrusions and complex dislocation structures, they form interface cracks and isolated dislocations during fatigue. These observations indicate a reduction in accumulated plastic strain in the films, which is likely a manifestation of their increased strength. In order to investigate thin film fatigue under conditions comparable to those present in sensors and microelectronic devices, very-high cycle fatigue behavior of supported Cu films with thicknesses between 40 and 360 nm have been investigated using a novel AFM-based resonance method. The films were deposited on AFM cantilevers which resemble a bending beam geometry and therefore show a gradient in strain amplitude. So that a single test suffices to provide data for strain amplitudes from zero up to a maximum of 0.22 % total strain. The different types of fatigue damage are investigated as a function of applied strain, film thickness and cycle numbers up to 5•10^10. Extrusions and grain boundary cracks are the dominant damage type for film thicker than 100 nm, but only appear above a threshold in the applied strain which scales inversely with the square root of the film thickness. The extrusion formation is attributed to dislocation activation. In films of 100 nm and thinner the fatigue damage is dominated by grain boundary grooves. They occur at a small threshold strain which decreases with the cycle number and therefore are believed to form by diffusion mediated creep processes. This indicates that thinner films can be less resistant to fatigue than thicker films, particularly for large cycle numbers.