Disclosed are method and apparatus for forming a thin layer. The method for forming the thin layer comprises providing a substrate including patterns, forming a bonding layer on the substrate covering an inner surface of a gap between the patterns, forming a preliminary layer on the bonding layer filling the gap; and thermally treating the preliminary layer to form the thin layer. The bonding layer is a self-assembled monomer layer formed using an organosilane monomer. The preliminary layer is formed from a flowable composition comprising polysilane.