Aluminum oxide (AlxOy) layers were deposited on polyethylene naphthalate substrates by low frequency plasma-enhanced atomic layer deposition process for barrier property enhancement. Trimethylaluminum and oxygen plasma were used as precursor and reactant materials, respectively. In order to enhance the barrier properties several process parameters were examined such as plasma power, working pressure and electrode-substrate distance. Increase of plasma power enhanced the reactivity of activated atomic and molecular oxygen to reduce the carbon contents in AlxOy layer, which appeared to enhance the barrier properties. But too high power caused generation of byproducts which were reincorporated in AlxOy layer to reduce the barrier properties. Plasma generated at lower working pressure was provided with an additional energy for reactions and had more diffusion of the plasma. The O/Al ratio of the layer approached the stoichiometric value by increasing the electrode-substrate distance. At the following conditions: 300 W of plasma power, 26.7 Pa of working pressure and 50 mm of electrode-substrate distance, water vapor transmission rates of the AlxOy layer reached 8.85 x 10(-4) g/m(2) day. (C) 2013 Elsevier B.V. All rights reserved.