Thin films made of amorphous silicon nitride (Si3N4) are usually deposited by chemical vapour deposition from silane (SiH4) and ammonia (NH3). Since they act as a barrier for water and sodium, they have a major role as passivation layers in microchip fabrication. Patterned nitride layers are also used as a mask for spatially selective silicon oxide growth, and as an etch mask when SiO2 masks cannot be used.

One example of the latter situation is given by the anisotropic etching of silicon in KOH. The etching rate of SiO2 in KOH is nearly 1000 times slower than the etching rate of silicon, and in most cases a SiO2 mask can be used successfully. However, a very deep selective etch may require a long etching time, and the 1000:1 etching rate ratio may result still too small to prevent the SiO2 mask from being etched off before the process is completed. In this circumstance Si3N4, thank to its reduced etched rate, can successfully replace the oxide mask layer.

The wet etching of nitride films is often performed in concentrated hot orthophosphoric acid (H3PO4). The bath temperature can range from 150°C to 180°C (boiling point) with a corresponding etch rate between 10 and 100 Å/min. It is good practice to bring the vapours into contact with a cold surface and to drive the condensed liquid back into the etching bath. This technique is referred to as "reflux".

The etching rates of silicon nitride, silicon oxide, and silicon in H3PO4 are respectively in the 50 : 5 : 1 ratio.