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

Abstract

Single Al 2 O 3 atomic layer deposition (ALD) films on polymers have demonstrated excellent gas diffusion barrier properties. Further improvements can be achieved using multilayers of Al 2 O 3 ALD layers with other inorganic layers. In this study, multilayers of Al 2 O 3 ALD and rapid SiO 2 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 Al 2 O 3 ALD films reduced the effective water vapor transmission rate (WVTR) to ∼1 × 10 -3 g/m 2 /day. When the Al 2 O 3 ALD film was directly exposed to the saturated H 2 O/HTO vapor pressure, the barrier properties deteriorated markedly after 4−5 days. Al 2 O 3 ALD barriers that were not subjected to direct H 2 O/HTO exposure indefinitely maintained low tritium transmission rates (TTRs). Rapid SiO 2 ALD layers deposited on the Al 2 O 3 ALD layer improved the diffusion barrier properties and protected the Al 2 O 3 ALD layers from H 2 O corrosion. The effective WVTR also reduced to ∼1 × 10 -4 g/m 2 /day for the Al 2 O 3 /SiO 2 ALD bilayer. Multilayers of Al 2 O 3 /SiO 2 bilayers initially further reduced the TTRs. Two Al 2 O 3 /SiO 2 bilayers reduced the effective WVTR to ∼5 × 10 -5 g/m 2 /day. Multilayers composed of >2 Al 2 O 3 /SiO 2 bilayers displayed degraded performance and effective WVTRs that were comparable with the single Al 2 O 3 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 Al 2 O 3 /SiO 2 bilayer and two Al 2 O 3 /SiO 2 bilayers could not be explained using laminate theory. The improvement suggests that the rapid SiO 2 ALD layer successfully closed pinhole defects in the Al 2 O 3 ALD layer.