Representative volume element of polycrystalline silicon for MEMS

In situ Atomic Force Microscopy (AFM) nanoscale deformation measurements resolved by Digital Image Correlation (DIG) were used to determine the representative volume element of polycrystalline silicon films for Microelectromechanical Systems (MEMS). Local displacement fields were recorded from 1×2 to 5×15-μm² areas in 2.5 μm thick freestanding specimens that were subject to uniform tension in order to determine the minimum size of the material domain whose mechanical behavior could be described by the isotropic constants. It was found that 10×10-μm² areas of polysilicon with 650 nm grain size containing on average 15×15 columnar grains constitute a representative volume element. On the contrary, displacement fields in 4×4 μm ² or 2×2 μm² polysilicon fields were highly inhomogeneous and the effective mechanical behavior of these specimen domains deviated from that described by isotropy. As a consequence, the isotropic material constants provide an accurate description of the mechanics of uniformly stressed MEMS components comprised of 15×15 or more grains.