Silicon nitride

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Silicon nitride
Identifiers
CAS number	[12033-89-5]
Properties
Molecular formula	N₄Si₃
Molar mass	140.28 g mol^-1
Appearance	grey, odorless powder
Density	3.44 g/cm³, solid
Melting point	1900 °C, 2173 K, 3452 °F (decomposes)
Hazards
EU classification	not listed
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Silicon nitride (Si₃N₄) is a hard, solid substance, that can be obtained by direct reaction between silicon and nitrogen at high temperatures. Silicon nitride is the main component in silicon nitride ceramics, which have relatively good shock resistance compared to other ceramics.

Rollers made of silicon nitride ceramic are sometimes used in high-end skateboard bearings, due to the material's shock and heat-resistant characteristics. It is also used as an ignition source for domestic gas appliances, hot surface ignition.

In microelectronics, silicon nitride is usually formed using chemical vapor deposition (CVD) method, or one of its variants, such as plasma-enhanced chemical vapor deposition (PECVD). It is usually used either as an insulator layer to electrically isolate different structures or as an etch mask in bulk micromachining. As a passivation layer for microchips, it is superior to silicon dioxide, as it is a significantly better diffusion barrier against water molecules and sodium ions, two major sources of corrosion and instability in microelectronics. It is also used as a dielectric between polysilicon layers in capacitors in analog chips.

Bulk, monolithic silicon nitride is used as a material for cutting tools, due to its hardness, thermal stability, and resistance to wear. It is especially recommended for high speed machining of cast iron. For machining of steel, it is usually coated by titanium nitride (usually by CVD) for increased chemical resistance.

[edit] Crystal Structure

There exist 3 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^[1].

See crystallographic structure of the α- and β- Si₃N₄ in [1] and γ phase Si₃N₄ in [2].

α- and β-Si₃N₄ have hexagonal structures, which are built up by corner-sharing SiN₄ tetrahedra. They can be regarded as consisting of layers of silicon and nitrogen atoms in the sequence ABAB... or ABCDABCD... in β-Si₃N₄ and α-Si₃N₄, respectively. The AB layer is the same in the α and β phases, and the CD layer in the α phase is related to AB by a c-glide plane. The Si₃N₄ tetrahedra in β-Si₃N₄ are interconnected in such a way that tunnels are formed, running parallel! with the c axis of the unit cell. Due to the c-glide plane that relates AB to CD, the α structure contains cavities instead of tunnels. The cubic γ-Si₃N₄ is often designated as c modification in the literature, in analogy with the cubic modification of boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms each coordinate six nitrogen atoms octahedrally, and one silicon atom coordinates four nitrogen atoms tetrahedrally. [3]