Novel Cyclosilazane-Type Silicon Precursor and Two-Step Plasma for Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride

We designed cyclosilazane-type silicon precursors and proposed a three-step plasma-enhanced atomic layer deposition (PEALD) process to prepare silicon nitride films with high quality and excellent step coverage. The cyclosilazane-type precursor, 1,3-di-isopropylamino-2,4-dimethylcyclosilazane (CSN-2), has a closed ring structure for good thermal stability and high reactivity. CSN-2 showed thermal stability up to 450 degrees C and a sufficient vapor pressure of 4 Torr at 60 degrees C. The energy for the chemisorption of CSN-2 on the undercoordinated silicon nitride surface as calculated by density functional theory method was -7.38 eV. The PEALD process window was between 200 and 500 degrees C, with a growth rate of 0.43 angstrom/cycle. The best film quality was obtained at 500 degrees C, with hydrogen impurity of similar to 7 atom %, oxygen impurity less than 2 atom %, low wet etching rate, and excellent step coverage of similar to 95%. At 300 degrees C and lower temperatures, the wet etching rate was high especially at the lower sidewall of the trench pattern. We introduced the three-step PEALD process to improve the film quality and the step coverage on the lower sidewall. The sequence of the three-step PEALD process consists of the CSN-2 feeding step, the NH3/N-2 plasma step, and the N-2 plasma step. The H radicals in NH3/N-2 plasma efficiently remove the ligands from the precursor, and the N-2 plasma after the NH3 plasma removes the surface hydrogen atoms to activate the adsorption of the precursor. The films deposited at 300 degrees C using the novel precursor and the three-step PEALD process showed a significantly improved step coverage of similar to 95% and an excellent wet etching resistance at the lower sidewall, which is only twice as high as that of the blanket film prepared by low-pressure chemical vapor deposition.