Role of Surface Termination in Atomic Layer Deposition of Silicon Nitride

There is an urgent need to deposit uniform, high-quality, conformal SiN_<i>x</i> thin films at a low-temperature. Conforming to these constraints, we recently developed a plasma enhanced atomic layer deposition (ALD) process with bis­(tertiary-butyl-amino)­silane (BTBAS) as the silicon precursor. However, deposition of high quality SiN_<i>x</i> thin films at reasonable growth rates occurs only when N₂ plasma is used as the coreactant; strongly reduced growth rates are observed when other coreactants like NH₃ plasma, or N₂–H₂ plasma are used. Experiments reported in this Letter reveal that NH_<i>x</i>- or H- containing plasmas suppress film deposition by terminating reactive surface sites with H and NH_<i>x</i> groups and inhibiting precursor adsorption. To understand the role of these surface groups on precursor adsorption, we carried out first-principles calculations of precursor adsorption on the β-Si₃N₄(0001) surface with different surface terminations. They show that adsorption of the precursor is strong on surfaces with undercoordinated surface sites. In contrast, on surfaces with H, NH₂ groups, or both, steric hindrance leads to weak precursor adsorption. Experimental and first-principles results together show that using an N₂ plasma to generate reactive undercoordinated surface sites allows strong adsorption of the silicon precursor and, hence, is key to successful deposition of silicon nitride by ALD