Nitrogen Self-diffusion in Polycrystalline Si_3N_4 Films: Isotope Heterostructures vs. Gas-exchange

摘要

The self-diffusion of nitrogen is investigated in polycrystalline thin silicon nitride films using a gas-exchange method (~(14)N_2/Si_3~(15)N_4) in comparison to Si_3~(14)N_4/Si_3~(15)N_4/Si_3~(14)N_4 isotope heterostructures. The films are produced by reactive r. f. magnetron sputtering. Depth profile analysis is carried out with secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS), and nuclear resonant reaction analysis (NRRA). The nitrogen diffusivities determined with the use of isotope heterostructures follow an Arrhenius law in the temperature range between 1200 and 1700 °C with an activation enthalpy of AH = 4.9 eV and a pre-exponential factor of D_0 = 1 * 10~(-6) m~2/s, indicating a conventional diffusion mechanism via localized point defects. Using the gas-exchange method, the nitrogen diffusivities could be obtained only in the temperature range between 1600 and 1700 °C. This is due to the fact that at temperatures below 1600 °C the surface exchange process with its high activation enthalpy (about 10 eV) is rate limiting, leading to non detectable diffusion profiles. The application of the different methods of depth profiling leads to the same diffusivities within estimated errors.