NOVELTY - The method involves introducing microwave into processing chamber (1) from planar antenna (31) through microwave transmitting plate (28), generating electric field in processing chamber, and generating plasma by exciting the processing. The silicon nitride film formed on target object is plasma-nitrided at processing temperature whose maximum is equal to film forming temperature in atomic layer deposition (ALD) method to form silicon nitride film modified by low-temperature nitrogen-containing plasma.    USE - Plasma-nitriding method used in manufacturing processes of semiconductor device for plasma-nitriding silicon nitride film by using plasma processing apparatus to obtain low-temperature nitrogen-containing plasma modified silicon nitride film employed for film in semiconductor device. Uses include but are not limited to spacer film, sidewall film and cap film in semiconductor device of e.g. dynamic random access memory (DRAM), logic device, or semiconductor memory device such as phase change random access memory (PRAM), resistive random access memory (ReRAM) and magnetoresistive memory. Can also be used in substrate for flat panel displays or substrate for solar cells.    ADVANTAGE - The plasma modification is effectively performed on silicon nitride film formed by low temperature ALD method at temperature that is equal to or smaller than film forming temperature in ALD method by using the plasma processing apparatus. Improves resistance to wet etching since nitrogen concentration of spacer film is increased and density of the film is increased by plasma-nitriding. Reduces thermal budget, maintains thermal resistance to heat generated in subsequent step and suppresses e.g. diffusion of atoms in heat sensitive semiconductor process by performing the plasma-nitriding process on silicon nitride film formed by low temperature ALD method at low temperature that is equal to or lower than its deposition temperature. Increases reliability of semiconductor device by employing for e.g. spacer film in semiconductor memory device e.g. DRAM.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross sectional schematic view of the configuration of a plasma processing apparatus.    Processing chamber (1)    Microwave transmitting plate (28)    Planar antenna (31)    Microwave generator (39)    Control unit (50)