Microtensile testing of free-standing and supported metallic thin films

Mechanical properties of free-standing and supported Cu thin films were investigated using a microtensile tester with in-situ strain measurement by optical diffraction. The pure metal films were deposited by electron beam deposition. The microstructure of the film was characterized by X-ray diffraction, electron microscopy and focused ion beam. The effect of annealing on the work hardening of free-standing Cu films was studied. A characteristic length similar to the average twin size was shown to be the dominant factor in work hardening. The anelastic deformation of annealed free-standing Cu films was investigated by stress relaxation and creep. The results were analyzed with a thermally activated dislocation glide mechanism. The measured activation volume is inversely proportional to the applied stress, consistent with the dislocation glide mechanism. Stress relaxation experiments were performed at temperatures between 20 and 80°C. An activation energy of approximately 0.1 eV was obtained and attributed to dislocation motion in the Peierls potential. The measured contribution from the dislocation mechanism can only account for a modulus deficit of 1--2%, not enough to explain the 10--20% deficit observed experimentally on thin films.; As-deposited Cu films with different film thicknesses on compliant polyimide substrates (Kapton) were tested with the microtensile tester. The modulus was independent of film thickness. The yield strength increased with decreasing film thickness. The variation in microstructure through the film thickness was measured by cross-sectional focused ion beam microscopy. The strengthening is attributed to the change in average twin size with thickness. Sputter-deposited Cu/Nb multilayers on polyimide (Kapton) tested in tension fractured before yielding.