This paper reports that amorphous silicon nitride(a-SiNx)overcoats were deposited at room temperature by microwave ECR plasma enhanced unbalanced magnetron sputtering.The 2 nm a-SiNx overcoat has better anti-corrosion properties than that of reference a-CNx overcoats(2-4.5 nm).The superior anti-corrosion performance is attributed to its stoichiomctric bond structure,where 94.8% Si atoms form Si-N asymmetric stretching vibration bonds.The N/Si ratio is 1.33 as in the stoichiometry of Si3N4 and corresponds to the highest hardness of 25.0 GPa.The surface is atomically smooth with RMS＜0.2 nm.The ultra-thin a-SiNx overcoats are promising for hard disks and read/write heads protective coatings.