Molecular layer deposition of nylon 66 films examined using in situ FTIR spectroscopy

Molecular layer deposition (MLD) techniques can utilize sequential, self-limiting surface reactions with bifunctional monomers to grow polymer films. In this study, transmission in situ Fourier transform infrared (FTIR) investigations were utilized to monitor the growth of nylon 66 MLD films using adipoyl chloride (AC) and 1,6-hexanediamine (HD) as the bifunctional monomers. In situ FTIR measurements on high surface area SiO2 powders monitored the linear growth of the integrated absorbance of the N-H, C-H, C-N, and CO stretching vibrations and the CO-N-H bending vibrations versus the number AC and HD sequential exposures. The growth of the integrated absorbance for the nylon 66 features was also self-limiting as a function of the individual AC and HD exposures. In addition to the vibrational modes expected for nylon 66, vibrational features were also observed that were consistent with some ammonium chloride salt incorporated in the film during nylon 66 MLD. The temperature dependence of the nylon 66 growth rate for constant exposures revealed that the highest growth rates were observed at the lowest sample temperature of 62 degrees C. This temperature dependence was consistent with a precursor-mediated adsorption model. Nylon 66 MLD was studied with and without surface functionalization of the SiO2 substrate using (3-aminopropyl)triethoxysilane (APS). The APS functionalization facilitated the reaction of AC with the amine-terminated surface. The growth rate of the nylon 66 MLD films was estimated using in situ transmission FTIR measurements on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbance of the C-H vibrational stretches was utilized to estimate the nylon 66 MLD growth rate per AC/HD reaction cycle. The observed growth rates were both above and below the expected growth rate for one nylon 66 bilayer. This range of growth rates illustrates the difficulties involved in using bifunctional monomers for MLD.