On mechanical properties of nanostructured meso-porous silicon

Abstract

Mechanical properties of meso-porous silicon are studied using topographic measurements and finite element simulations. Our approach is based on an original analysis of the strain at the free surface of porous silicon tub embedded in bulk Si regions allowing the determination of the Young’s modulus of the porous layers. In particular, the internal stress in the porous Si region is evaluated from the corresponding deformation of the monocrystalline Si adjacent region which mechanical parameters are well known. Moreover, a mechanical anisotropy of the columnar nanostructured porous Si is brought to the fore from the characteristic shape of the strained porous layer profile. Moderately oxidized, 70% in porosity, porous silicon patterns were investigated. Correlation of our measurements with x-ray data reported early in literature shows the macroscopic strain being close to the silicon lattice relative increase revealing an elastic deformation regime. The porous layers exhibit an unexpected low and strongly anisotropic Young’s modulus for all samples. Young’s modulus values of 1.5 and 0.44 GPa are found in parallel and perpendicular directions of the columnar structure, respectively. Finally, a phenomenological model for such a mechanical behavior taking into account porosity and percolation strength factor of the randomly arranged as-prepared and partially oxidized porous Si nanostructures is proposed.