By entering a low acceleration Si ion beam of 500 V or lower or a low acceleration Si ion beam of 500 V-2000 V having been slanted such that an injection depth becomes shallow, which has been mass-separated from a liquid alloy ion source containing Si by a mass separator and converged by an ion optical system, the amplitude defect near a surface of the Mo/Si multilayer film or the Mo.sub.2C/Si multilayer film is removed by a physical sputter or a gas assist etching such that an interlayer of the Mo/Si multilayer film or the Mo.sub.2C/Si multilayer film in a lower layer is not destroyed.