Low-temperature plasma coating of electroluminescence particles with silicon nitride film

A Si₃N₄ thin film was deposited on ZnS phosphor particles of 18μm diameter in a silane-nitrogen radio-frequency (r.f.) plasma at 310-330 K. The particles were frequently shaken to maintain contact with plasma gas. The film deposited was characterized by X-ray photoelectron spectroscopy XPS, Fourier-transformed infrared spectroscopy and high-resolution scanning electron microscopy (SEM). The Si/N ratio of the film was about 1.25, and little change in the infrared spectrum was observed following the exposure in the ambient air for 10 days. The performance of the film as a diffusion barrier was evaluated by monitoring the lifetime of electroluminescence (EL). A film as thin as 50-60 nm could cover the EL powder without pinholes, and successfully protected the phosphor from water vapour.