Paper TF+VT-WeM2
Measuring the Diffusion Coefficient of Water in High Quality Permeation Barrier Layers

Wednesday, October 30, 2013, 8:20 am, Room 104 A

Deposited thin film permeation barriers are of great interest for the protection of flexible organic light-emitting diode (OLED) displays. Water can permeate on three pathways into such barrier coated devices: 1. Diffusion through the bulk of the barrier; 2. Permeation through defects in the barrier; and 3. Permeation along the barrier/substrate interface. During research on new barrier materials, permeation through defects and along interfaces can easily dominate permeation through the bulk of the barrier. This makes evaluation of the inherent permeability of a new barrier material difficult. We present two fast turnaround techniques that measure the diffusion of water in flexible barrier films that have ultra-low permeability: electrical capacitance, and mechanical stress. These techniques are calibrated against the results of secondary ion mass spectrometry (SIMS). Capacitance is measured on layers sandwiched between electrodes on glass substrates. Stress is measured from the curvature of barrier films deposited on silicon wafers. Both the capacitance and the stress technique measure very low diffusion coefficients. All three techniques are applied to one specific type of ultra-low permeability barrier film prepared by the plasma oxidation of hexamethyl disiloxane. Films are exposed to water between 65°C and 200°C, including D₂O and H₂O¹⁸ for SIMS. From the accelerated diffusion data we extrapolate barrier lifetimes at room temperature. Lifetime is defined as the time required for the permeation of one monolayer of water.

SIMS provides values of H, D, O¹⁶ and O¹⁸ concentrations and their depth profiles. We use these to calibrate the concentrations in the capacitance and stress measurements, and to identify the diffusion mechanism. Capacitance is highly sensitive to the in-diffusion of water because of its high dielectric constant (liquid H₂O: 80 vs. SiO₂: 3.9). Mechanical stress also is highly sensitive because the barrier swells as water diffuses in. Each exposure to water produces a change of capacitance and stress from which diffusion coefficients are extracted. We assume that capacitance and stress are linear with water content of the barrier.

For samples exposed to water at 100°C, the diffusion coefficient determined from SIMS is 4.4x10^-15cm²/s, from capacitance 5.6x10^-15cm²/s, and from stress 4.2x10^-15cm²/s. The close agreement suggests that any one of the three techniques will yield reliable results. The activation energy for the diffusion coefficient is 0.7 eV and that for water solubility is -0.2 eV. At 30°C and 100% relative humidity, one monolayer of H₂O will diffuse through a 2.5 micrometer thick barrier layer in 20 years.