Stress relaxation in Si-rich silicon nitride thin films

Si-rich silicon nitride thin films have been deposited by low pressure chemical vapor deposition, at 850 °C from mixtures of dichlorosilane and ammonia. The films’ elastic properties have been studied as a function of film composition. Fourier transform infrared spectroscopy and ellipsometric data indicate that the local atomic strain is a strong function of the calculated volume fraction of Si contained in the films. A relationship is observed that shows the strain to be inversely proportional to the cube root of the Si volume fraction. A model that accounts for distortion in Si–SixN4−x tetrahedra (x=0–4), upon substitution of silicon for nitrogen in the film is applied to the data. The model is shown to be consistent with measurements of intrinsic film stress across a compositional range from stoichiometric silicon nitride, Si3N4, to nitrogen-free amorphous silicon, a-Si.