Ion-assisted deposition (IAD) is one of the most widely used techniques for the deposition of thin films. Electron cyclotron resonance (ECR) ion sources are ideal for ion-assisted deposition due to high ionization efficiency even at low pressures. Silicon nitride films have been deposited by rf reactive sputtering with ion assistance from an ECR plasma. The ECR power was varied from 0 to 250 W. The effect of ECR plasma on the growth of silicon nitride films has been shown to be systematic and is characterized by using Fourier- transform infrared spectroscopy and atomic force microscopy. The surface roughness for the films deposited without ECR plasma was about 2 nm and decreased to about 0.7 nm for those deposited with 250 W of ECR power. The optical band gap changed from 2.2 to 4.9 eV as the ECR power was increased, indicating higher reactivity in the presence of ECR plasma. Good quality silicon nitride films with a band gap of 4.9 eV, a refractive index of 1.92, and an extinction coefficient of 4 X 10(-4) have been obtained with an ECR power of 100 W (corresponding ion density of 1.5 x 10(10) cm(-3)) and the surface roughness was less than 0.7 nm. The compositional analysis carried out using Auger electron spectroscopy showed a Si/N ratio of 0.73 for the films deposited under optimum deposition conditions.