Silicon Nitride - Production

Production

The material is prepared by heating powdered silicon between 1300 °C and 1400 °C in an atmosphere of nitrogen:

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, for example, 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 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 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 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.

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)

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:

1. 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
2. Plasma-enhanced chemical vapor deposition (PECVD) technology, which works at rather low temperature and vacuum conditions.

The lattice constants of silicon nitride and silicon are different. Therefore 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).