Silicon nitride

Silicon nitride
Identifiers
CAS number	[12033-89-5]
PubChem	3084099
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)
Related compounds
Other anions	silicon carbide, silicon dioxide
Other cations	boron nitride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Silicon nitride (Si₃N₄) is a chemical compound of silicon and nitrogen. It is a hard ceramic having high strength over a broad temperature range, moderate thermal conductivity, low coefficient of thermal expansion, moderately high elastic modulus, and unusually high fracture toughness for a ceramic. This combination of properties leads to excellent thermal shock resistance, ability to withstand high structural loads to high temperature, and superior wear resistance. Silicon nitride is mostly used in high-endurance and high-temperature applications, such as gas turbines, car engine parts, bearings and metal working and cutting tools. Silicon nitride bearings are used in the main engines of the NASA's Space shuttles. Thin silicon nitride films are a popular insulating layer in silicon-based electronics and silicon nitride cantilevers are parts of atomic force microscopes.

History

Si₃N₄ gas turbine engine radial rotor

Silicon nitride was first produced in 1857 by Deville and Wohler,^[1] but remained merely a chemical curiosity. It took almost hundred years to bring it to applications. During 1948-1952, the Carborundum Company, Niagara Falls, New York, applied for several patents on the manufacture and application of silicon carbide. By 1958 Haynes (Union Carbide) silicon carbide was in commercial production for thermocouple tubes, rocket nozzles, and boats and crucibles for melting metals. British work on silicon nitride, started in 1953, was aimed at high-temperature parts of gas turbines and resulted in development of reaction-bonded silicon nitride and hot-pressed silicon nitride. In 1971, the Advanced Research Project Agency of the US Department of Defense has placed a US$ 17 million contract with Ford and Westinghouse for two ceramic gas turbines.^[2]

Synthesis

Silicon nitride can be obtained by direct reaction between silicon and nitrogen at high temperatures:

3 Si + 2N₂ → Si₃N₄

by carbon-assisted nitridation:

3 SiO₂ + 6 C + 2 N₂ → Si₃N₄ + 6 CO

or by diimide synthesis:

SiCl₄ + 6 NH₃ → Si(NH)₂ + 4 NH₄Cl

3 Si(NH)₂ → Si₃N₄ + 2 NH₃

Electronic-grade silicon nitride is usually formed using chemical vapor deposition (CVD), or one of its variants, such as plasma-enhanced chemical vapor deposition (PECVD):

3 SiH₄ + 4 NH₃ → Si₃N₄ + 12 H₂.

For deposition of silicon nitride layers on semiconductor substrates, two methods are used:
1) LPCVD-technology (low pressure chemical vapor deposition), which works at rather high temperature and is done either in a vertical or in a horizontal tube furnace.^[3]
2) PECVD-technology (plasma enhanced chemical vapor deposition), 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.^[4]

Silicon nitride nanowires can also be produced by sol-gel method.^[5]

Natural existence of silicon nitride is restricted to meteorites, where it very rarely occurs as mineral nierite.

Processing

Silicon nitride is difficult to produce as a bulk material because it cannot be heated over 1850 °C due to its dissociation and therefore is difficult to sinter by conventional hot press sintering techniques. Bonding of silicon nitride powders can be achieved through adding additional materials (sintering aids or "binders") which commonly induce a degree of liquid phase sintering.^[6] A cleaner alternative is to use spark plasma sintering where heating is achieved 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.^[7][8]

Crystal structure and properties

Blue atoms are nitrogen and gray are silicon atoms

trigonal α-Si3N4.

hexagonal β-Si3N4

cubic γ-Si3N4

There exist three crystallographic structures of silicon nitride (Si₃N₄), designated as α, β and γ phases.^[9] 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^[10][11].

The α- and β-Si₃N₄ have trigonal and hexagonal structures, respectively, 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.^[8]

The longer stacking sequence results in the α-phase having higher hardness than the β-phase. However, the α-phase is chemically unstable compared with the β-phase. At high temperatures when a liquid phase is present, the α-phase always transforms into the β-phase. Therefore, β-Si₃N₄ is the major form used in Si₃N₄ ceramics.^[12]

Applications

In general, the issue with applications of silicon nitride has not been technical performance, but cost. As the cost has come down, the number of production applications is accelerating.^[13]

Automobile industry

One of the major applications of sintered silicon nitride is in automobile industry as a material for engine parts. Those include, in Diesel engines, glowplugs for faster start-up; precombustion chambers (swirl chambers) for lower emissions, faster start-up and lower noise; turbocharger for reduced engine lag and emissions. In spark-ignition engines, silicon nitride is used for rocker arm pads for lower wear, turbocharger for lower inertia and less engine lag, and in exhaust gas control valves for increased acceleration. As examples of production levels, there is an estimated 300,000 sintered silicon nitride turbochargers made annually.^[6][13]

Bearings

Inner rings of Si₃N₄ bearings

Silicon nitride ceramics have good shock resistance compared to other ceramics. Therefore, ball bearings made of silicon nitride ceramic are used in performance bearings. A representative example is use of silicon nitride bearings in the main engines of the NASA's Space Shuttle.^[14][15]

Silicon nitride ball bearings are harder than metal which reduces contact with the bearing track. This results in 80% less friction, 3-10 longer lifetime, 80% higher speed, 60% smaller weight, ability to operate with lubrication starvation, higher corrosion resistance and higher operation temperature, as compared to traditional metal bearings.^[13] Silicon nitride ball bearings can be found in high end automotive bearings, industrial bearings, wind turbines and even sometimes in high-end skateboards. Silicon carbide bearings are especially useful in applications where corrosion, electric or magnetic fields prohibit the use of metals. For example, in tidal flow meters where seawater attack is a problem or in electric field seekers.^[6]

Si₃N₄ was first demonstrated as a superior bearing in 1972 but did not reach production until nearly 1990 because of challenges associated with reducing the cost. Since 1990, the cost has been reduced substantially as production volume has increased. Although Si₃N₄ bearings are still 2–5 times more expensive than the best steel bearings, their superior performance and life are justifying rapid adoption. Around 15–20 million Si₃N₄ bearing balls were produced in the U.S. in 1996 for machine tools and many other applications. Growth is estimated at 40% per year, but could be even higher if ceramic bearings are selected for consumer applications such as in-line skates and computer disk drives.^[13]

High-temperature material

Silicon nitride thruster. Left: Mounted in test stand. Right: Being tested with H₂/O₂ propellants

Silicon nitride has long been used in high-temperature applications. In particular, it was identified as one of the few monolithic ceramic materials capable of surviving the severe thermal shock and thermal gradients generated in hydrogen/oxygen rocket engines. To demonstrate this capability in a complex configuration, NASA scientists used advanced rapid prototyping technology to fabricate a one-inch-diameter, single-piece combustion chamber/nozzle (thruster) component. The thruster was hot-fire tested with hydrogen/oxygen propellant and survived five cycles including a 5-min cycle to a 1320 °C material temperature.^[16]

Metal working and cutting tools

The first major application of Si₃N₄ was abrasive and cutting tools. Grinding, milling, and boring of metals is a major cost of manufacturing. A study in the early 1970s estimated that there were 2,692,000 metal-cutting machine tools in the United States with an annual operating cost of $64 billion.^[13]

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. Hot hardness, fracture toughness and thermal shock resistance mean that sintered silicon nitride can cut cast iron, hard steel and nickel based alloys with surface speeds up to 25 quicker than those obtained with conventional materials such as tungsten carbide.^[6] The use of Si₃N₄ cutting tools has had a dramatic effect on manufacturing output. For example, face milling of gray cast iron with silicon nitride inserts doubled the cutting speed, increased tool life from one part to six parts per edge, and reduced the average cost of inserts by 50%, as compared to traditional tungsten carbide tools.^[13]

Electronics

Example of local silicon oxidation through a Si₃N₄ mask

Silicon nitride is often used as an insulator and chemical barrier in manufacturing integrated circuits, 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.

The following two reactions deposit nitride from the gas phase:

3SiH₄ + 4NH₃ → Si₃N₄ + 12H₂

3SiCl₂H₂ + 4NH₃ → Si₃N₄ + 6HCl + 6H₂

Si₃N₄ AFM cantilever

Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile stress, which may crack films thicker than 200 nm. However, it has higher resistivity and dielectric strength than most insulators commonly available in microfabrication (10¹⁶ Ω·cm and 10 MV/cm, respectively).

Another two reactions may be used in plasma to deposit SiNH:

2SiH₄ + N₂ → 2SiNH + 3H₂

SiH₄ + NH₃ → SiNH + 3H₂

These films have much less tensile stress, but worse electrical properties (resistivity 10⁶ to 10¹⁵ Ω·cm, and dielectric strength 1 to 5 MV/cm).^[17]

Silicon nitride is also used in xerographic process as one of the layer of the photo drum.^[18] Silicon nitride is also used as an ignition source for domestic gas appliances.^[19] Because of its good elastic properties, silicon nitride, along with silicon and silicon oxide, is the most popular material for cantilevers -- the sensing elements of atomic force microscopes.^[20]

References

Further reading

• R. C. Sangster (2005). Formation of Silicon Nitride: From the 19th to the 21st Century - Materials Science Foundations (monograph series). Volumes 22 - 24. Trans. Tech. Publications. ISBN 0-87849-492-8. 
• V.I. Belyi and L.L. Vasilyeva (1988). Silicon Nitride in Electronics. Elsevier, Nederlands. ISBN 10: 0444426892. 
• G. Ziegler et al. (1987). "Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride". Journal of Materials Science 22: 3041-3086. doi:10.1007/BF01161167. 
• S. Veprek et al. (1995). "A concept for the design of novel superhard coatings". Thin Solid Films 268: 64. doi:10.1016/0040-6090(95)06695-0. 
• Silicon Nitride Ceramics Overview Article and free download