Amorphous silicon nitride and silicon carbonitride films were deposited by the plasma immersion ion processing technique. The SiCxNy films were obtained in the range of compositions 0less than or equal tox/(x + y)less than or equal to0.53. Gas pressure during the deposition was kept around 0.13 Pa (1 mTorr) and SiH4, N-2, Ar, and C2H2 gas mixtures were used. The composition of the film grown without addition of C2H2 was close to that Of Si3N4. With the addition Of C2H2 the areal density of Si and N atoms remains almost constant, whereas the concentration of C increases. The addition of carbon to the films causes a decrease in hardness from 13.5 to 0.85 GPa, a reduction in the band gaps from about 5 to about 2 eV, and a decrease in the refractive index from about 1.63 to 1.5. The density of the films with the addition of carbon decreases from 2.37 to 0.46 g/cm(3). The evolving physical properties in these films are shown to result from both the increasing fraction of C2H2 in the gas mixture and a reduction in the average energy imparted to the growing film during deposition. (C) 2003 American Vacuum Society.