Abstract: A method of forming an insulation film on a semiconductor substrate by plasma enhanced atomic layer deposition (PEALD), includes: (i) adsorbing a non-excited non-halide precursor having four or more silicon atoms in its molecule onto a substrate placed in a reaction space; (ii) supplying an oxygen-free reactant to the reaction space without applying RF power so as to expose the precursor-adsorbed substrate to the reactant; and (iii) after step (ii), applying RF power to the reaction space while the oxygen-free reactant is supplied in the reaction space; and (iv) repeating steps (i) to (iii) as a cycle, thereby depositing an insulation film on the substrate.