A plasma based ion implantation (PBII) to Si wafers with N ions is performed and the surface modification of Si, i.e., the compositional and structural changes at the surface, are examined by energy dispersive X-ray spectrometer (EDX), Raman and Fourier Transform Infrared (FT-IR) spectroscopy, Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) measurements and X-ray photoelectron spectroscopy (XPS). To check ion species in N 2 plasma, optical emission spectroscopy is also carried out. The implantation time is varied from 10 min to 7 h. It is found that the N concentration is increased with increasing implantation time up to 1 h, and then tends to be saturated at further implantation time. The results of Raman measurements indicate that the small peaks from a-Si and/or a-SiN x appear after N ion implantation in addition to the Raman peak from Si crystal. It is also confirmed by FT-IR and XRD measurements that a broad peak assigned to Si-N bonds appears and no crystal phases of Si 3 N 4 is observed after N ion implantation. That the maximum N areal density is approximately 1.7 x 10 17 atoms/cm 2 is obtained for the sample implanted for 7 h. The XPS results show that implanted N mostly bonds to Si, and the ratio of N/Si at the surface is slightly increased over 1 h until 7 h. Judging from these results, it is suggested that a-SiN x is formed on Si wafer by N ion implantation using PBII system.