PEALD of SiO and AlO Thin Films on Polypropylene: Investigations of the Film Growth at the Interface, Stress, and Gas Barrier Properties of Dyads.

A study on the plasma-enhanced atomic layer deposition of amorphous inorganic oxides SiO and AlO on polypropylene (PP) was carried out with respect to growth taking place at the interface of the polymer substrate and the thin film employing in situ quartz-crystal microbalance (QCM) experiments. A model layer of spin-coated PP (scPP) was deposited on QCM crystals prior to depositions to allow a transfer of findings from QCM studies to industrially applied PP foil. The influence of precursor choice (trimethylaluminum (TMA) vs [3-(dimethylamino)propyl]-dimethyl aluminum (DMAD)) and of plasma pretreatment on the monitored QCM response was investigated. Furthermore, dyads of SiO/AlO, using different Al precursors for the AlO thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO and AlO from TMA on scPP is significantly hindered if no oxygen plasma pretreatment is applied to the scPP prior to depositions, the DMAD process was found to yield comparable AlO growth directly on scPP similar to that found on a bare QCM crystal. From this, the interface formed between the AlO and the PP substrate is suggested to be different for the two precursors TMA and DMAD due to different growth modes. Furthermore, the residual stress of the thin films influences the barrier properties of SiO/AlO dyads. Dyads composed of 5 nm AlO (DMAD) + 5 nm SiO exhibit an oxygen transmission rate (OTR) of 57.4 cm m day, which correlates with a barrier improvement factor of 24 against 5 when AlO from TMA is applied.