Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by Al₂O₃ and Rapid SiO₂ Atomic Layer Deposition

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Abstract

Single Al2O3 atomic layer deposition (ALD) films on polymers have demonstrated excellent gas diffusion barrier properties. Further improvements can be achieved using multilayers of Al2O3 ALD layers with other inorganic layers. In this study, multilayers of Al2O3 ALD and rapid SiO2 ALD were grown on Kapton and heat-stabilized polyethylene naphthalate substrates. Transmission rates for tritium through the films were measured using the radioactive HTO tracer method. Comparison with previous Ca tests and tritium exchange experiments with alcohols indicated that the tritium in HTO may diffuse as either molecular HTO or atomic tritium. Assuming that the tritium diffuses only as HTO, single Al2O3 ALD films reduced the effective water vapor transmission rate (WVTR) to ∼1 × 10-3 g/m2/day. When the Al2O3 ALD film was directly exposed to the saturated H2O/HTO vapor pressure, the barrier properties deteriorated markedly after 4−5 days. Al2O3 ALD barriers that were not subjected to direct H2O/HTO exposure indefinitely maintained low tritium transmission rates (TTRs). Rapid SiO2 ALD layers deposited on the Al2O3 ALD layer improved the diffusion barrier properties and protected the Al2O3 ALD layers from H2O corrosion. The effective WVTR also reduced to ∼1 × 10-4 g/m2/day for the Al2O3/SiO2 ALD bilayer. Multilayers of Al2O3/SiO2 bilayers initially further reduced the TTRs. Two Al2O3/SiO2 bilayers reduced the effective WVTR to ∼5 × 10-5 g/m2/day. Multilayers composed of >2 Al2O3/SiO2 bilayers displayed degraded performance and effective WVTRs that were comparable with the single Al2O3 ALD film. These multilayer barriers may have cracked during handling and mounting as a result of brittleness at larger thicknesses. The barrier improvement observed for one Al2O3/SiO2 bilayer and two Al2O3/SiO2 bilayers could not be explained using laminate theory. The improvement suggests that the rapid SiO2 ALD layer successfully closed pinhole defects in the Al2O3 ALD layer.

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