Silicon nitride is used in integrated circuits wherever a high dielectric material is required. We are most used to silicon dioxide as the dielectric used in IC fabrication. Silicon nitride has some properies that makes it a better dielectric sometimes. One of the chief ones is its use in the fabrication of higher value on chip capacitance. Its dielectric constant is approximately twice that of silicon dioxide. So a sandwich capacitor made using nitride will be twice as large in the same area as it would be if oxide was used. Common properties of nitride are listed in a table located at http://www.signalpro.bz/nitride.pdf.