The influence of oxide and adsorbates on the nanomechanical response of silicon surfaces

In this article we report the influence of surface oxides and relative humi dity on the nanomechanical response of hydrophobic and hydrophilic Si surfa ces. Depth-sensing nanoindentation combined with force modulation enabled m easurement of surface forces, surface energy, and interaction stiffness pri or to contact. Several regimes of contact were investigated: pre-contact, a pparent contact, elastic contact, and elasto-plastic contact. Both humidity and surface preparation influenced the surface mechanical properties in th e pre- and apparent-contact regimes. Meniscus formation was observed for bo th hydrophobic and hydrophilic surfaces at high humidity. Influence of humi dity was much less pronounced on hydrophobic surfaces and was fully reversi ble. In the elastic and elasto-plastic regimes, the mechanical response was dependent on oxide layer thickness. Irreversibility at small loads (300 nN ) was due to the deformation of the surface oxide. Above 1 mu N, the deform ation was elastic until the mean contact pressure reached 11 GPa, whereby S i underwent a pressure-induced phase transformation resulting in oxide laye r pop-in and breakthrough. The critical load required for pop-in was depend ent on oxide thickness and tip radius. For thicker oxide layers, substrate influence was reduced and plastic deformation occurred within the oxide fil m itself without pop-in. Elastic modulus and hardness of both the oxide lay er and Si substrate were measured quantitatively for depths <5 nm.