In the through Silicon via (TSV) process as a key technology for the 3-dimensional LSI, particularly in the ‘via-last process', SiN_x films of high density are strongly required to prepare at low deposition temperatures. It, however, is generally known that the SiN_x film prepared at low temperatures shows low film density, resulting in poor insulating barrier properties. As a solution of this issue, we propose the use of the SiN_x films deposited by reactive sputtering. We can obtain the sputtered SiN_x films of high density (2.78∼2.99 g/cm³) in spite of the deposition without substrate heating. The 20-nm-thick SiN_x films succeed the experimental check on the barrier properties against Cu diffusion upon annealing at 700°C for 1 h. The films also show good step-coverage for a TSV with an aspect ratio of 1.5. The SiN_x films prepared by reactive sputtering are a candidate for a good insulating barrier applicable to the via-last TSV process.