Effect of Backbone Chemistry on the Structure of Polyurea Films Deposited by Molecular Layer Deposition

An experimental investigation into the growth of polyurea films by molecular layer deposition was performed by examining trends in the growth rate, crystallinity, and orientation of chains as a function of backbone flexibility. Growth curves obtained for films containing backbones of aliphatic and phenyl groups indicate that an increase in backbone flexibility leads to a reduction in growth rate from 4 to 1 angstrom/cycle. Crystallinity measurements collected using grazing incidence X-ray diffraction and Fourier transform infrared spectroscopy suggest that some chains form paracrystalline, out-of-plane stacks of polymer segments with packing distances ranging from 4.4 to 3.7 angstrom depending on the monomer size. Diffraction intensity is largely a function of the homogeneity of the backbone. Near-edge X-ray absorption fine structure measurements for thin and thick samples show an average chain orientation of similar to 25 degrees relative to the substrate across all samples, suggesting that changes in growth rate are not caused by differences in chain angle but instead may be caused by differences in the frequency of chain terminations. These results suggest a model of molecular layer deposition-based chain growth in which films consist of a mixture of upward growing chains and horizontally aligned layers of paracrystalline polymer segments.