On the intrinsic moisture permeation rate of remote microwave plasma-deposited silicon nitride layers

We report on a low substrate temperature (110 °C) remote microwave plasma-enhanced chemical vapor deposition (PECVD) process of silicon nitride barrier layers against moisture permeation for organic light emitting diodes (OLEDs) and other moisture sensitive devices such as organic photovoltaic cells (OPVs). Specifically, the influence of the SiH4/NH3 gas flow ratio on the layer composition and intrinsic moisture barrier performance is investigated, as inferred from Fourier Transform Infrared (FTIR) spectroscopy, Rutherford Back Scattering (RBS) analysis, and the calcium test. Since the presence of extrinsic factors for barrier failure such as pinholes and contamination particles (defects) is largely determined by the substrate conditioning and environment, the focus in this research is on the intrinsic permeability of the silicon nitride films, as measured by monitoring the homogeneous degradation of defect (e.g. pinholes and contamination particles)-free calcium regions. The investigated films have tunable chemical composition and optical properties and moderate residual strain levels varying from tensile to compressive. Despite this variation in film properties, the intrinsic water vapor transmission rate (WVTR) is found to be constant at a level of 1 · 10- 5 g/m2 day at 20 °C/50%RH conditions for films deposited at 0.1–0.5 nm/s. When the total gas flow rate is increased in order to achieve higher growth rate processing (0.2–1.0 nm/s), a higher permeability is measured at increased SiH4/NH3 ratio. The development of high surface roughness in the silicon nitride layer, as shown by AFM analysis, suggests cluster/dust formation in the plasma and powder inclusion during film deposition. This eventually leads to higher water vapor transmission rate of the deposited layers.