(705h) 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.