A study of ta-C, a-C:H and Si-a:C:H thin films on polymer substrates as a gas barrier

Abstract

Two types of diamond-like carbon (DLC) films were grown on Poly (ethylene terepthalate) (PET) substrates have been investigated for density, internal stress, gas permeability and structural properties by an X-ray reflectometry, surface profilometer, Raman spectroscopy and water vapour permeation analysis system, respectively. The high density tetrahedral amorphous carbon (ta-C) films (3.27 g/cm 3) prepared by filtered vacuum cathodic arc (FCVA) showed unexpected high water vapour transmission rate (WVTR) (1.3 g/m 2 day) and a surface covered by a network of deep micro-cracks, which is due to intrinsic stress inside the ta-C films (up to 12 GPa). The soft Si doped hydrogenated amorphous carbon (Si-a:C:H) films prepared by plasma enhanced chemical vapour deposition (PECVD) exhibited low transmission rate (0.03 g/m 2 day) with a water vapour reduction factor up to 98% and a surface almost completely free of micro-cracks. Si incorporation in the a-C:H films reduced both the film density from 2.3 g/cm 3 to 1.85 g/cm 3 and the compressive stress to <0.5 GPa. This could be understood by two possibilities. Firstly, the increasing in the hydrogen content within the films (as indicated by increasing the Raman background slope) developed more polymer–like bonds, which weakens the microstructure. Second, replacing the stronger CC bonds (3.7 eV) by CSi (3.21 eV) bonds where the relaxation of residual stress would occur with large strains in the CSi bonds.