PEALD of SiO₂ and Al₂O₃ 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 2 and Al 2 O 3 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 2 /Al 2 O 3, using different Al precursors for the Al 2 O 3 thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO 2 and Al 2 O 3 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 Al 2 O 3 growth directly on scPP similar to that found on a bare QCM crystal. From this, the interface formed between the Al 2 O 3 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 2 /Al 2 O 3 dyads. Dyads composed of 5 nm Al 2 O 3 (DMAD) + 5 nm SiO 2 exhibit an oxygen transmission rate (OTR) of 57.4 cm 3 m –2 day –1, which correlates with a barrier improvement factor of 24 against 5 when Al 2 O 3 from TMA is applied.