The roles of hydrophobic group on the surface of ultra low dielectric constant porous silica film during thermal treatment

Porous silica films with ultra low-k (below 2) and low leakage current densities (10− 8 A/cm2 or lower at an electric field of 1.8 MV/cm) were prepared by the surfactant-template method. Hexamethyldisilazane (HMDS), a surface modification agent, was utilized to yield hydrophobic groups on the surface of porous silica film to prevent the absorption of moisture. It effectively retained the low permittivity properties of the films. Thermal treatment at high temperature (> 350 °C) destroyed surface hydrophobic groups and generated hydrophilic groups (Si–OH), which replaced the surface Si(CH3)3 groups, and resulted in the absorption of moisture. However, Si–OH not only resulted in the absorption of moisture but also initiated the formation of trimethylsilyl groups on the surface by HMDS. When the damaged film is repaired by HMDS again, the k value falls to its initial value (which may be below 1.6). A denser hydrophobic low-k film is formed and the electrical properties are improved.