A one-dimensional fluid model is developed to investigate the behavior of plasma in the afterglow of pulsed voltage modulated SiH4/N-2/NH3 discharges at radio-frequency (400 kHz), which are used for the deposition of silicon nitride (SiNx) films. The model incorporates particle balances, electron energy balance, and Poisson's equation, allowing the equations solved self-consistently. The current work is an attempt to understand the effect of duty cycle on the characteristics of silicon nitride (SiNx) films. Therefore, the influences of duty cycle on the plasma density, electron temperature, and ion energy at the powered electrode were carefully discussed. We find that by decreasing the duty cycle, the negative ions can be able to escape during the off-time, and the positive ion energy is effectively decreased. On the other hand, the positive ion density is slightly decreased. (C) 2011 Elsevier B.V. All rights reserved.