Low dielectric constant silica films with ordered nanoporous structure

Low-dielectric-constant (low-k) materials are believed to be necessary to allow speed increases in integrated circuits. These materials are desirable to reduce the RC time constant of interconnect structures and to reduce the cross-talk between adjacent lines. A number of intensive researches have been conducted in this area, including non-fluorinated polymers, inorganic–organic hybrids, porous polymer and porous silica. Among them porous silica films are considered to be the most promising ones, because they usually have good chemical and thermal stability and are compatible with the silicon wafer and related materials used in existing IC technology. In this paper, a novel route to prepare low-k nanoporous SiO2 films is reported. Silicate sols are prepared with the precursor Si(OC2H5)4 and surfactant cetyltrimethyl ammonium bromide (CTAB) catalyzed by hydrochloric acid. The films are derived from dip-coating process. The refractive index and dielectric constant are measured with an ellipsometer and impedance analysis apparatus, which are smaller than 1.15 and 2.5 respectively. XRD and TEM patterns show that silica films derived from a mole ratio of CTAB/TEOS of 0.10 or 0.15 have typical mesoporous structures with a period of 5 nm and a pore size of 4 nm. A drag-wipe test shows that films with ordered porous structure have much better mechanical properties than those with disordered porosity. And hydrophobicity of silica films can be achieved by HMDS treatment and the contact angle of water to the surface of hydrophobic films can be up to 140°.