Evaluation on plastic deformation property of copper nano-film by nano-scale cantilever specimen

We investigate the elasto-plastic deformation properties of a 20-nm-thick copper (Cu) thin film. A nano-scale cantilever specimen is fabricated from multilayer thin films, where the Cu thin film is sandwiched between a silicon nitride layer and a silicon substrate. During bending, the load, P, and displacement, d, are carefully monitored using an electron microscope, and a distinct non-linearity is observed. The plastic constitutive equation of the Cu thin film, which is assumed to obey a power hardening law (σ = Rεn (σ > σy)), is inversely derived by finite element method fitting the experimental results. The residual stress in each layer is experimentally examined, and the effect is included in the inverse analysis. We obtain σ = 3316ε0.29 [MPa] and a yield stress of 765 MPa for the Cu film. The yield stress is about 10 times higher than that of the bulk, and the exponent is also larger. Moreover, inverse analysis based on the bending experiment data, without considering the residual stress, gives a good approximation of the plastic law. This is because the plastic deformation preferentially takes place at the top and bottom surfaces, where the residual stress is relieved during fabrication of the specimen.