Effect of Backbone Flexibility on the Structure and Orientation of Polyurea Chains Grown By Molecular Layer Deposition

Effect of Backbone Flexibility on the Structure and Orientation of Polyurea Chains Grown by Molecular Layer Deposition

 

David S. Bergsman^1,*, Richard G. Closser², Christopher J. Tassone³, Bruce M. Clemens⁴, Dennis Nordlund^3, and Stacey F. Bent^1,2,4

¹Department of Chemical Engineering, ²Department of Chemistry, ⁴Department of Materials Science and Engineering, Stanford University, Stanford, California, USA 94305. ³SLAC National Accelerator Laboratory, Menlo Park, California 94025

* bergsdav@stanford.edu

In many nanotechnology applications, such as microprocessors, sensors, and solar cells, there is an increasing need for the incorporation of nanoscale polymeric films. However, many commonly-used polymer deposition processes cannot meet the increasingly strict requirements regarding film thickness, conformality, composition, and crystallographic structure needed for these technologies. One method of meeting these demands is molecular layer deposition (MLD), in which polymer chains are grown from the vapor phase by exposing a surface to an alternating sequence of two or more monomers that only react with the previous monomer. Due to the self-limiting nature of these reactions, films deposited using this technique have been shown to be conformal on even high aspect ratio substrates, and the library of materials that can be deposited is continuing to expand. But despite the continuing advancements made in the development of this technique, many questions still remain about the mechanism behind the film growth and their resulting structures. In order to develop a better model for its growth behavior, MLD was used to deposit polyurea films onto silicon with bifunctional monomers whose backbone chemistries were varied in conformational flexibility. Backbones included phenyl, ethyl, and butyl groups for both the first and second monomers used in the A-B process. Trends in the thickness, bonding, crystallinity, and chain orientation were then measured using variable-angle spectroscopic ellipsometry, Fourier transform infrared spectroscopy, grazing incidence x-ray diffraction, and angle-dependent near edge X-ray absorption fine structure. These trends were then used to deduce structural information about the chains in these films. Growth rates of these chemistries ranged from about 4 angstroms/cycle for the phenyl-phenyl backbone to 1 angstrom/cycle for the butyl-butyl backbone, suggesting that the films primarily adopt non-fully extended configurations. Fourier transform infrared spectroscopy confirms the polymerization of the monomers, as well as demonstrating increased ordering of the urea groups for the phenyl-phenyl, butyl-butyl, and ethyl-butyl backbones. Grazing incidence x-ray diffraction further supports this ordering, and shows an out-of-plane paracrystalline peak with lattice spacing that decreases with increasing chain flexibility. Combined with molecular orientation data collected with x-ray absorption, it is believed that these chains form mixed domains, with some chains lying in horizontally stacks of paracrystalline segments and some chains adopting tilted configuration, growing away from the substrate. The implications of these results on the likelihood of double reaction terminations occurring will also be discussed.