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T. Walter:
"High cycle fatigue properties of Cu films";
Betreuer/in(nen): C. Eisenmenger-Sittner, G. Khatibi; Institut für Festkörperphysik, 2014; Abschlussprüfung: 25.11.2014.

In this study mechanical fatigue behaviour of different types of Cu metallization film stacks (consisting of Si/SiO2/TiW/Cu) on Si substrates was investigated in the high cycle fatigue regime at room temperature. The influence of thickness, grain size and texture as well as the composition of the diffusion barrier on the deformation behaviour of Cu metallization layers in the thickness range of 5 µm to 20 µm was studied. The samples, which consisted of Cu-metallized Si strips were subjected to symmetrical tension - compression fatigue loading by using an ultrasonic resonance fatigue testing systems working at 36kHz at room temperature. Systematic investigations of surface degradation and fatigue damage in the films showed distinct slip band formation in preferentially oriented grains. Evaluation of fatigue life and the degree of damage was based on the determination of the slip band density as a function of loading cycles as measured by means of optical microscopy. The degree of damage was found to be highly dependent on film thickness, grain orientation and grain size. Moreover it was found that there exists a trend between the adhesion strength of Si/diffusion barrier/Cu and the degree of surface damage during the high frequency fatigue loading (higher adhesion strength of the diffusion barriers to the silicon substrate resulted in a higher degree of surface deformation of the Cu films). This result may be used as a measure for the evaluation of the dynamic adhesion response of thin film stacks on Si substrates. Furthermore, in order to study the effect of constraint on the lifetime and cyclic deformation behavior of the Cu films, free standing miniaturized bar shaped fatigue specimens were prepared by chemical removal of the silicon substrate. Fatigue life curves (Wöhler curves) were determined by using an ultrasonic fatigue system working at 20kHz and a special set-up suitable for testing of free standing miniaturized specimens. The comparison of fatigue tests on free standing Cu bars with Cu film stacks of the same thickness and microstructure showed that cyclic induced plastic deformation is noticeably suppressed in constraint films. Cu bars showed a higher density of deformed grains, with rougher and broader extrusions compared to the features observed in Cu films on a silicon substrate. The fatigue life curve of the free standing Cu bars covered a stress range of 15-25MPa in the high cycle regime (>106), while early stages of slip band formation on the surface of Cu metallization on Si was observed first at stress levels in the range of 72MPa. The results of this study are a contribution to understanding the mechanical fatigue response of constrained metallization films on substrates and provides a new efficient methodology.