Modelling creep induced by internal stresses in freestanding submicron Cu film

Abstract

In a lab-on-chip experiment, lithography and selective chemical etching are used to pattern microscopic tensile test samples within a thin metallic layer hosting large internal stresses. After partial release of the layer from the substrate on which it was deposited, the free-standing beam-like structures are stretched by the actuator to which they are connected. The lab-on-chip also comprises cantilever beams which shorten freely upon release from the substrate. Experimental observations of both the instantaneous and the delayed deformations in a 170 nm thick copper film were simulated using a theoretical model. The model properly reproduced the experiments only when accounting for both plasticity and significant kinematic hardening occurring already during the deposition of the polycristalline film. Once released from the substrate, cantilever beams contracted well beyond the elastic range because the amplitudes of back-stresses were sufficient to cause reverse plastic yielding. Large tensile stresses inside the actuated beams led to delayed uniform elongations (creep) exceeding 16%. Such values are much larger than the uniform strain of 5-6% that was observed in beams that underwent necking as soon as the film was released from the substrate, i.e. directly after etching of the sacrificial layer.