It was well known that the interaction of solid surface with low energy ion beam results in the phenomena of sputtering and implantation. The characteristics of thin film Si3N6 formed by implanting ions N2+ at energy 1350 eV into single crystal Si are studied in this paper.We consider that it is necessary to introduce the concept of effective dose for ion beam which depends on the thickness of damage layer and the etching rate of as-implanted silicon nitrides.