Silicon nitride

Silicon nitride is a compound of silicon and nitrogen, Si₃N₄. In bulk it's a very hard ceramic used for bearings and cutting tools. In electronics it appears as clear films a fraction of a micrometer thick that block electricity, moisture and stray ions.

On display glass it's made in the same kind of plasma chamber as amorphous silicon. Silane supplies the silicon and ammonia supplies the nitrogen, often with extra nitrogen gas. The plasma breaks the gases apart, and silicon and nitrogen atoms join on the warm glass. A film made this way holds some hydrogen and doesn't match the formula exactly, so engineers write it SiNₓ. Tuning the gas mix adjusts its properties.

In each thin-film transistor, a silicon nitride layer sits between the gate and the silicon channel. It's the gate insulator: thin enough to let the gate's voltage reach through, solid enough that no current leaks. Across the whole thin-film transistor array, another layer covers the finished transistors and wiring, protecting them from moisture and from the liquid crystal. A handy trick: the insulator and the silicon channel can be deposited one after the other without breaking vacuum, which keeps the boundary between them clean.