Effect of Pore Structure of Nanometer Scale Porous Films on the Measured Elastic Modulus

The impact of pore structure of nanoporous films on the measured elastic modulus is demonstrated for silica-based nanoporous low-k films that are fabricated using an alternative manufacturing sequence which allows a separate control of porosity and matrix properties. For this purpose, different experimental techniques for measuring the elastic properties were compared, including nanoindentation, laser-induced surface acoustic wave spectroscopy (LAwave), and ellipsometric porosimetry (EP). The link between the elastic response of these nanoporous materials and their internal pore structure was investigated using positronium annihilation lifetime spectroscopy (PALS), EP, and diffusion experiments. It is shown that the absolute value of the Berkovich indentation modulus is very sensitive to the local pore structure and stiffness of the substrate and can be influenced by densification and/or anisotropic elasticity upon indentation, while on the other hand spherical indentation results are less sensitive to the local pore structure. The comparison of Berkovich and spherical indentation results combined with finite element simulations can potentially reveal changes in the internal structure of the film. For nanoporous films with porosity above the percolation threshold, the elastic modulus results obtained with LAwave and EP agree very well with spherical indentation results. On the other hand, below the percolation threshold, the elastic modulus values determined by these techniques deviate from the spherical indentation results. This was explained in terms of specific technique related effects that appear to be sensitive to the specific arrangement and morphology of the pores.