Sodium diffusion in plasma-deposited amorphous oxygen-doped silicon nitride (<i>a</i>-SiON:H) films

Abstract

The dependence of 22Na diffusivity in a-SiN:H and a-SiON:H films on hydrogen content, N/Si ratio, oxygen content, and temperature was determined. Measurements were made at 350, 375, and 400 °C. The diffusion profiles for sodium in a-SiN:H films have two regions; a short Gaussian region and shallow linear region. We have interpreted these results using a structural model which describes the film as a two-phase mixture of a highly ordered Si3N4-like phase and a highly disordered phase. Using this model we argue that the Gaussian region of the diffusion profile is due to ‘‘bulk’’ diffusion through the Si3N4-like phase and the shallow linear region is due to ‘‘fast’’ diffusion through the interfacial region. At 400 °C the ‘‘fast’’ diffusivity is on the order of 103 times larger than the ‘‘bulk’’ diffusivity. However, at 80 °C the ‘‘fast’’ diffusion is approximately 105 times smaller than the bulk diffusivity. This large change in the ratio of the diffusion coefficients is due to the large difference in activation energy for the two mechanism. For ‘‘bulk’’ diffusion it is approximately 1.8 eV, whereas it is approximately 2.6 eV for ‘‘fast’’ diffusion. The high activation energy for ‘‘fast’’ diffusion is due to an interaction (association) of the diffusing Na+ ion with a (N-Si-N)− group to form a Na-N-Si-N complex. The association energy for this complex is approximately 1.2 eV. We have observed only one region in the diffusion profiles in the oxygen-doped films. This region is due to ‘‘bulk’’ diffusion. The magnitude and activation energy for diffusion is strongly dependent upon the oxygen content. It decreases from approximately 1.8 to 1.3 eV when the oxygen content of the film increases from 0% to 25%. These results are then used to discuss possible device reliability problems which may occur in oxygen-doped films.