Experimental results on freestanding nanoscale Al films

We present a new methodology, based on MEMS (micro electro mechanical systems) technology, that allows to apply known forces on material samples of free standing thin films (20nm and higher) and measure corresponding displacements of the samples in-situ in SEM or TEM. We demonstrate the methodology by carrying out two experiments: (a) uniaxial tensile experiments on 30-200nm thick, 20µm wide and 200µm long free standing aluminum samples. A unique feature of this experimental approach is that the sample and the loading instrument are co-fabricated so that the alignment of sample and loading is within 10E-5 degrees - a critical concern in a uniaxial test particularly at small scale. The stress strain response of the samples indicates: (1) the modulus of elasticity is in the range 62-75GPa, lower values for thinner samples, (2) yielding occurs at 327MPa, 710MPa, 328MPa and 250MPa in 200nm, 100nm, 50nm and 30nm thick samples showing negative Hall-Petch behavior, (3) after yielding, there is a short strain hardening regime followed by an almost ideal plastic behavior for thicker films, whereas for thinner films (30 and 50nm) the behavior is nearly brittle. (b) Bending experiments on 100-200nm thin Al films to explore the effect of strain gradients on strain hardening of nanocrystalline materials. It is found that the length parameter related to the strain gradient theories proposed in the literature approaches zero when the film thickness approaches 100nm, but for 200nm film, it is around 4 micro meter.