Experimental evidences of competing stress relaxation mechanisms in thin Al/Si and Pd films tested on chip

Coulombier, Michaël [UCL] Guisbiers, Grégory [UCL] Colla, Marie-Stéphane [UCL] Raskin, Jean-Pierre [UCL] Pardoen, Thomas [UCL]

A novel stress relaxation technique dedicated to thin freestanding films has been developed as an extension of an on-chip tensile testing method. The technique relies on a structure made of two beams. An actuator beam is deposited with large internal stresses onto a sacrificial layer. The specimen beam is deposited next, partly overlapping with the actuator beam. By etching the sacrificial layer, the two-beam structure is released from the substrate and the test specimen is deformed until force equilibrium is reached. The specimen, being under stress, relaxes at a rate depending on the composition, microstructure, thickness, and temperature. This test is similar to a creep test on a specimen attached to a spring. The strain rate sensitivity exponent and activation volume can be derived from a record of the evolution with time of the displacement undergone by the test specimen. This technique allows measuring the relaxation response of a large amount of test structures, involving different levels of plastic strains, dimensions and loading conditions, without requiring the use of any external device such as a nanoindentor or tensometer. The technique is well suited for very long relaxation measurements. Strain rates varying between 10-6 s-1 down to 10-11 s-1 are accessible and statistically representative data can be generated. The technique has been applied to both 205 nm-thick evaporated Al 1%Si and 355 nm-thick evaporated Pd films showing interesting variations of the activation volume with strain rate and initial level of plastic deformation. Relaxation at room temperature has been studied as well as at higher temperature for the Pd films. In the AlSi films, the initial activation volume amounts to a few b3 presumably due to diffusion mechanisms. The diffusion mechanism requires stress gradients which smooth out during relaxation, leaving room, at lower strain rates, to another dominant relaxation mechanism involving a much larger activation volume. The recovery of the dislocation structure is one of the candidates for this second mechanism. These mechanisms are investigated using TEM.