14 Externe wissenschaftliche Einrichtungen

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    Fatigue of Al thin films at ultra high frequencies
    (2005) Eberl, Christoph; Arzt, Eduard (Prof. Dr. phil.)
    Ultra high-cycle fatigue at frequencies in the GHz regime leads to a characteristic void and extrusion formation in patterned metal thin films. Resulting from the microstructural damage formation a significant degradation in form of a shift of the resonance frequency and failures by short circuits in Surface Acoustic Wave (SAW) test devices take place. To study fatigue at ultra high cycles, SAW test devices were used to test continuous and patterned Al thin films at ultra high frequencies. For stress amplitudes as low as 14 MPa lifetime measurements showed no fatigue limit for 400 nm Al thin films. The resulting damage sites appeared in regions of cyclic stress concentration as identified by Finite Element Analysis. In situ measurements revealed that the characteristic extrusion/void formation mechanism operates on a short time scale. The post-test analysis of microstructural changes reveals extrusion and void formation concentrated at grain boundaries. This finding and the observed grain growth indicates a high material flux at the grain boundaries induced by the cyclic load. Quantitative analysis also shows a correlation between extrusion density and electrical devices performance. This direct correlation shows a functional agreement with a common theory on the influence of crack density on intrinsic stresses in thin metal films. Advanced Finite Element (FEM) calculations simulate very well the sensitivity of the resonance frequency to damage structure in interconnects such as cracks, voids and extrusions. The experimentally observed linear correlation between damage density and frequency shift is reproduced by the FEM model. The estimation of the short circuit probability from the extrusion length distribution revealed an exponential dependency on the electrode distance. The observed damage formation is explained by the combined action of dislocation motion and stress-induced diffusion processes.