With the method according to an embodiment of the present invention, by introducing alcohol such as ethylene glycol, 1,2 propanediol and isopropyl alcohol (IPA), a hydrocarbon such as ethylene containing at least one unsaturated bond, N2, or ether such as diethyl ether as a secondary source gas, linear oligomers are bridged and the mechanical strength of a film can be improved while a low dielectr