| The strong adherence (stiction) of adjacent surfaces is a major design concern in microelectromechanical systems (MEMS). Analysis of the different stiction micro-mechanisms and the elastic and elastic-plastic deformation of asperities at MEMS interfaces is developed using two- and three-dimensional fractal descriptions of surface topography which are scale-invariant. Expressions for the elastic and plastic components of the normal contact force and real contact area are derived in terms of fractal parameters, material properties, and mean surface separation distance. The influence of surface roughness, relative humidity, applied voltage, and material properties on the magnitude of the van der Waals, electrostatic, and capillary forces is analyzed in light of simulation results. It is shown that the effects of surface roughness and applied voltage on the maximum stiction force are significantly more pronounced than that of material properties. Results for the critical pull-off stiffness versus surface roughness are presented for different material properties and microstructure stand-free surface spacing.; Single and repeated indentation of dynamic face-centered-cubic (fcc) Lennard-Jones (LJ) and metallic substrates by rigid tips are investigated using three-dimensional molecular dynamics (MD) simulations. Force hysteresis is observed in each indentation loading-unloading cycle. The generation of a step dislocation in a L-J solid is revealed by a single atom indentation simulation. Results show that the compressive yield strength, elastic unloading stiffness, and energy dissipation decrease with increasing substrate equilibrium temperature and decreasing indentation speed. The significance of the interfacial atomic potential and tip shape on the deformation behavior is elucidated by comparing results for copper indented by a blunt fcc copper tip and a sharp hydrogen-terminated diamond tip. The formation of a connective neck during unloading is observed for the copper-copper system but not for the diamond-copper system due to the stronger interatomic forces and larger tip area in the former system. Results for copper and silver substrates repeatedly indented by a hydrogen-terminated diamond tip up to a fixed depth or maximum normal force are presented in order to illustrate the evolution of deformation and heating in the substrate with indentation cycles. Behaviors resembling cyclic hardening and softening are revealed in the MD studies. |
