A silicon-containing insulation film is formed on a substrate by plasma reaction using a reaction gas including (i) a source gas comprising a silicon-containing hydrocarbon compound containing multiple cross-linkable groups, (ii) a cross-linking gas, and (iii) an inert gas, into a reaction chamber where a substrate is placed. The insulation film is then exposed to electron beam radiation, thereby increasing mechanical strength of the film without substantial alternation of its dielectric constant.