Plasma-deposited silicon oxide barrier films on polyethersulfone substrates: Temperature and thickness effects

Silicon oxide (SiO_x) films deposited on flexible polyethersulfone (PES) substrates by plasma-enhanced chemical vapor deposition (PEVCD) have been investigated for transparent barrier applications. Although the water vapor transmission rate (WVTR) of PES (∼28 g/m²/day; thickness: 200 μm) is higher than that of the polyethylene terephthalate (PET; ∼16 g/m²/day; thickness: 188 μm), the PES substrate can withstand process temperatures of up to 180 °C, providing more flexibility in the design of device processing. Details of the substrate-temperature and film-thickness effects on the SiO_x/PES properties in terms of transmittance, refractive index, deposition rate, adhesion, roughness, and WVTR were described. When the substrate temperature increased from 80 to 170 °C, the deposition rate, adhesion, and roughness values were found to increase while the WVTR decreased to a value of near 0.3 g/m²/day at 150 °C. With increasing the oxide thickness from 50 to 500 nm, the surface roughness increased from 2.71 to 5.84 nm. A lower WVTR value can be achieved under a barrier thickness of 200 nm. Further improvement was carried out by depositing a 100-nm-thick SiO_x film on both sides of the PES substrate, which resulted in a minimum WVTR of 0.1 g/m²/day. The double-sided coatings on PES could balance the stress and greatly improve the WVTR data.