Molecular layer deposition of poly(p-phenylene terephthalamide) films using terephthaloyl chloride and phenylenediamine

Ultrathin polymer films can be fabricated using the gas phase method known as molecular layer deposition (MLD). This process typically uses bifunctional monomers in a sequential, self-limiting reaction sequence to grow conformal polymer films with molecular layer control. In this study, terephthaloyl chloride (TC) and phenylenediamine (PD) were used as the bifunctional monomers to deposit poly(p-phenylene terephthalamide) (PPTA) thin films. 3-aminopropyl trimethoxysilane (APMS) or ethanolamine (EA) was used to functionalize the surface to prepare an amine-terminated surface prior to the PPTA MLD. The surface chemistry and growth rate during PPTA MLD at 145°C were studied using in situ transmission Fourier transform infrared (FTIR) spectroscopy experiments on high surface area powders of SiO2 particles. PPTA MLD thin film growth at 145°C was also examined using in situ transmission FTIR experiments on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbance of the N-H and amide I stretching vibrations was measured and used to determine the thin film thickness. X-ray reflectivity (XRR) experiments were also employed to measure the film thickness after PPTA MLD at 145°C and 180°C. The experiments revealed that TC and PD reactions displayed self-limiting surface chemistry. The surface species alternated with sequential TC and PD exposures and the PPTA MLD films grew continuously. However, the growth rates per MLD cycle were less than the ideal expectations and varied between 0.4-2.9 A per TC/PD reaction cycle. The lower growth rates are explained by the growth of a limited number of polymer chains on the substrate. The variability in the growth rate is attributed to the difficulties with the bifunctional monomer precursors. Alternative surface chemistries for polymer MLD are proposed that would avoid the use of bifunctional monomers.