Mechanical Behavior Of Nanolayered Cu/W Thin Films Under Cyclic And/Or Monotonous Biaxial Solicitations: In-Situ Study By X-Ray Synchrotron Diffraction

Mechanical properties of thin films are of utmost interest for a large variety of technological applications such as stretchable microelectronics. The mechanical behavior of thin films depends on their microstructure (grain texture, size, defects…). In order to mimic their actual in-use loading conditions, we performed controlled biaxial deformation tests on composites formed by metallic thin films deposited on polyimide substrate (Kapton^®). Here, we present the results obtained for an equi-biaxial deformation test performed on two Cu/W nanocomposite thin films in order to analyze the influence of residual stresses. These thin films elaborated by ion-beam sputtering are about 150 nm thick (7 x (18 nm Cu + 6 nm W). Film residual stresses were obtained in either tension or compression controlling the gaz pressure in the deposition chamber (the evaluated intra-granular stresses are relatively large: +2.0 GPa in tension and -3.6 GPa in compression).

To perform the mechanical experiments, we used the biaxial tensile setup of the DiffAbs beamline at synchrotron SOLEIL [1]. During mechanical testing, the applied and film in plane strains are measured simultaneously by both digital image correlation (macroscopic strains) and synchrotron X-ray diffraction (lattice strains) techniques [2].

Comparing the two charts, one can make the following observations: Ø  First, the accumulated elastic deformation in the sample prepared in compression is much higher than that obtained for the sample in tension. This is not surprising since we can intuiter that by undergoing a tensile stress to a sample that is already claimed, the amount of energy that can be stored in elastic form is much less than for an initially compressed sample. What is surprising, however, is that for a residual stress state voltage of about 2 GPa, it can be applied an elastic deformation of about 0.4% for the Tungsten (and 0,2% for the Copper) whereas for a residual stress state of compression of about -4GPa elastic deformation can be applied in the order of 0.8% for the Tungsten (and 0.4% for the Copper). This is the result of the combined effects of the microstructure and confinement by the substrate. Ø  Secondly, it is noted that, in both cases, the tungsten is elastically deformed much more (by a factor 2 for each branch) than Copper. Moreover, after unloading, the Copper sub-layers return in compression regardless of the initial state. So it seems that the Copper deforms plastically. In the case of Tungsten prepared in tension and despite the presence of a plate (a sign of irreversible deformation), we don’t observe the compression discharge. So it seems that the Tungsten has a brittle behavior in this case, as previously observed by Soundès Djaziri [3].

To conclude, what appears through these graphics is the significant effect of residual stresses on the mechanical behavior of Cu/W multilayers. Indeed, the presence of compressive stresses in the sample delays the plasticity event (plateau for Copper) and allows for the Copper and Tungsten sublayers to support elastic strains twice as large.

References

[1] G. Geandier, D. Thiaudière, et al., Rev. Sci. Instrum. 81 (2010) 103903.

[2] S. Djaziri, P.-O. Renault, F. Hild, E. Le Bourhis, Ph. Goudeau, D. Thiaudière, D. Faurie, J. Appl. Cryst. 44 (2011) 1071-1079.

[3] Djaziri. S, Elasticité et endommagement sous chargement bi-axial de nano-composites W/Cu en couches minces sur polyimide : apport des techniques synchrotrons, Thèse de l’université de Poitiers, 2012