On the atomic mechanisms of water-enhanced silicon wafer direct bonding

Abstract

The quantum-chemical methods of modified neglect of diatomic overlap (MNDO) and Austin model 1 (AM1) are discussed and applied to the investigation of the different steps of direct bonding of silicon or silica wafers enhanced by water molecules. Concerning the hydrophilization, the reaction enthalpy of joining a water molecule to the silica surface was found to be 4.3 kcal mol −1, which is insufficient for a molecule to disintegrate, therefore higher concentrations of OH − ions are necessary to form silanol bonds, realized, e.g., by NH 4OH treatment. The strength of the H 2O interaction with an oxidized surface is nearly five times higher than that with pure silicon. At low bond densities, i.e., if only one siloxane bond is formed from two adjacent silanol groups, the transformation of silanol bonds into siloxane ones is exothermal, leading to an easy procedure, implying, however, low adhesion (fixing). Stronger bonding is provided by higher densities of reacting silanol bonds, but then the process becomes endothermal because of spatial restrictions, requiring a thermal treatment. At the hydrophilized silica surface a specific arrangement of silanol bonds and water clusters results. Its thermodynamic stability is determined by the equilibrium of core-core repulsion and the exchange interaction of the electron shells. As the formation enthalpy depends non-linearly on the bond number the stable configuration is attained at a density of four silanol bonds per 100 Å 2, coupled with 2–4 water molecules per silanol bond.