Surface reaction mechanisms for atomic layer deposition of silicon nitride

We use density functional theory to investigate atomic layer deposition of silicon nitride using silane and ammonia as precursors. Cluster models are used to represent the reactive sites on the surface of silicon nitride. We have simulated surface reactions on as-deposited Si3N4 as well as the initial Si3N4 growth on Si(1 0 0). We have calculated the kinetics and mechanisms for the reactions of SiH4 and NH3 on Si–H∗ and N–H∗ surface sites. We find that reactions of NH3 and SiH4 with the Si–H∗ surface sites of as-deposited Si3N4 have activation barriers of 40 and 80 kcal/mol, respectively. On the other hand, SiH4 and NH3 react on the N–H∗ surface sites of as–deposited Si3N4 with activation barriers of 52 and 114 kcal/mol, respectively. The low activation barriers for forming Si–N bonds relative to the barriers for reactions that form Si–Si and N–N bonds lead to the properties of selectivity and self-limiting reaction required for ALD. The barrier for formation of Si–Si bonds is significantly below the barrier for formation of N–N bonds explaining the tendency for CVD grown Si3N4 films using SiH4 and NH3 to be Si rich.