Molecular Layer Deposition of Alucone Polymer Films Using Trimethylaluminum and Ethylene Glycol

Polymeric films can be grown by a sequential, self-limiting surface chemistry process known as molecular layer deposition (MLD). The MLD reactants are typically bifunctional monomers for stepwise condensation polymerization and can yield completely organic films. The MLD of organic–inorganic hybrid polymers can also be accomplished using a bifunctional organic monomer and a multifunctional inorganic monomer. In this work, the growth of a poly(aluminum ethylene glycol) polymer is demonstrated using the sequential exposures of trimethylaluminum (TMA) and ethylene glycol (EG). These hybrid polymers, known as alucones, were grown over a wide range of temperatures from 85 to 175 °C. In situ quartz crystal microbalance and ex situ X-ray reflectivity experiments confirmed linear growth of the alucone film versus number of TMA/EG reaction cycles at all temperatures. The alucone growth rates decreased at higher temperatures. Growth rates varied from 4.0 Å per cycle at 85 °C to 0.4 Å per cycle at 175 °C. In situ Fourier transform infrared spectroscopy was used to monitor the surface reactions during alucone MLD. Ex situ FTIR spectroscopy, X-ray photoelectron spectroscopy, and X-ray reflectivity measurements were also employed to determine the chemical composition, thickness, and density of the alucone films. These ex situ studies revealed that the alucone films grown on Al2O3 ALD surfaces evolved under ambient conditions before reaching a stable state. Alucone films capped with rapid SiO2 ALD displayed much more stability than alucone films grown on Al2O3 ALD surfaces. These results indicated that H2O may facilitate the chemical transformation of the alucone MLD films. The alucone films represent a new class of organic–inorganic hybrid polymers. Modification of this basic alucone MLD chemistry with use of other diols or other bifunctional monomers can produce different alucone polymers with variable properties.