A multilayer resist process comprises forming in sequence an undercoat film, an intermediate film, and a photoresist film on a patternable substrate, and effecting etching in multiple stages. A silicon-containing film forming composition is useful in forming the intermediate film serving as an etching mask, comprising a silicon-containing polymer obtained through hydrolytic condensation of at least one Si—Si bond-containing silane compound having formula: R(6-m)Si2Xm wherein R is a monovalent hydrocarbon group, X is alkoxy, alkanoyloxy or halogen, and m is 3 to 6. The composition allows the overlying photoresist film to be patterned to a satisfactory profile and has a high etching selectivity relative to organic materials.