Silicon nitride is a chemical compound of the elements silicon and nitrogen. Si3N4 (trisilicon tetranitride) is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It is a white, high-melting-point solid that is relatively chemically inert, being attacked by dilute HF and hot H3PO4. It is very hard (8.5 on the Mohs scale). It has a high thermal stability with strong optical nonlinearities for all-optical applications.
Production Silicon nitride is prepared by heating powdered silicon between 1300 °C and 1400 °C in a nitrogen atmosphere:
3 Si + 2 N2 → Si3N4 The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen. Without an iron catalyst, the reaction is complete after several hours (~7), when no further weight increase due to nitrogen absorption (per gram of silicon) is detected. In addition to Si3N4, several other silicon nitride phases (with chemical formulas corresponding to varying degrees of nitridation/Si oxidation state) have been reported in the literature. These include the gaseous disilicon mononitride (Si2N), silicon mononitride (SiN) and silicon sesquinitride (Si2N3), each of which are stoichiometric phases. As with other refractories, the products obtained in these high-temperature syntheses depends on the reaction conditions (e.g. time, temperature, and starting materials including the reactants and container materials), as well as the mode of purification. However, the existence of the sesquinitride has since come into question. It can also be prepared by the diimide route:
SiCl4 + 6 NH3 → Si(NH)2 + 4 NH4Cl(s) at 0 °C 3 Si(NH)2 → Si3N4 + N2 + 3 H2(g) at 1000 °C Carbothermal reduction of silicon dioxide in a nitrogen atmosphere at 1400–1450 °C has also been examined:
3 SiO2 + 6 C + 2 N2 → Si3N4 + 6 CO The nitridation of silicon powder was developed in the 1950s, following the "rediscovery" of silicon nitride and was the first large-scale method for powder production. However, use of low-purity raw silicon caused contamination of silicon nitride by silicates and iron. The diimide decomposition results in amorphous silicon nitride, which needs further annealing under nitrogen at 1400–1500 °C to convert it to a crystalline powder; this is now the second-most-important route for commercial production. The carbothermal reduction was the earliest used method for silicon nitride production and is now considered as the most-cost-effective industrial route to high-purity silicon nitride powder.
Film deposition Electronic-grade silicon nitride films are formed using chemical vapor deposition (CVD), or one of its variants, such as plasma-enhanced chemical vapor deposition (PECVD):
3 SiH4(g) + 4 NH3(g) → Si3N4(s) + 12 H2(g) at 750–850°C 3 SiCl4(g) + 4 NH3(g) → Si3N4(s) + 12 HCl(g) 3 SiCl2H2(g) + 4 NH3(g) → Si3N4(s) + 6 HCl(g) + 6 H2(g) For deposition of silicon nitride layers on semiconductor (usually silicon) substrates, two methods are used:
Low pressure chemical vapor deposition (LPCVD) technology, which works at rather high temperature and is done either in a vertical or in a horizontal tube furnace, or Plasma-enhanced atomic layer chemical vapor deposition (PECVD) technology, which works at rather low temperature (≤ 250 °C) and vacuum conditions. Examples include bis(diethylamino)silane as silicon precursor and plasma of N2 as reactant. Since the lattice constants of silicon nitride and silicon are different, tension or stress can occur, depending on the deposition process. Especially when using PECVD technology this tension can be reduced by adjusting deposition parameters. Silicon nitride nanowires can also be produced by sol-gel method using carbothermal reduction followed by nitridation of silica gel, which contains ultrafine carbon particles. The particles can be produced by decomposition of dextrose in the temperature range 1200–1350 °C. The possible synthesis reactions are:
SiO2(s) + C(s) → SiO(g) + CO(g) and 3 SiO(g) + 2 N2(g) + 3 CO(g) → Si3N4(s) + 3 CO2(g) or 3 SiO(g) + 2 N2(g) + 3 C(s) → Si3N4(s) + 3 CO(g).
Processing Silicon nitride is difficult to produce as a bulk material—it cannot be heated over 1850 °C, which is well below its melting point, due to dissociation to silicon and nitrogen. Therefore, application of conventional hot press sintering techniques is problematic. Bonding of silicon nitride powders can be achieved at lower temperatures through adding materials called sintering aids or "binders", which commonly induce a degree of liquid phase sintering. A cleaner alternative is to use spark plasma sintering, where heating is conducted very rapidly (seconds) by passing pulses of electric current through the compacted powder. Dense silicon nitride compacts have been obtained by this techniques at temperatures 1500–1700 °C.
Crystal structure and properties
There exist three crystallographic structures of silicon nitride (Si3N4), designated as α, β and γ phases. The α and β phases are the most common forms of Si3N4, and can be produced under normal pressure condition. The γ phase can only be synthesized under high pressures and temperatures and has a hardness of 35 GPa.
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