Silicon nitride

General
Surname	Silicon nitride
other names	SN SSN GPSSN HPSN HIPSN RBSN
Ratio formula	Si 3 N 4
Brief description	pale beige solid
External identifiers / databases
CAS number 12033-89-5 EC number 234-796-8 ECHA InfoCard 100,031,620 PubChem 3084099 Wikidata Q413828
properties
Molar mass	140.28 g mol −1
Physical state	firmly
density	3.44 g cm −3
Sublimation point	1900 ° C (sublimation)
solubility	almost insoluble in water
safety instructions
GHS labeling of hazardous substances Caution H and P phrases H: 335 P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Silicon nitride balls

Silicon nitride (also: silicon nitride) is a chemical compound that is used as a component of a technical material. It consists of the elements silicon and nitrogen , has the formula Si ₃ N ₄ and belongs to the class of nitrides .

In nature, silicon nitride is known as the rare mineral nierite , which has so far only been found in meteorites .

Crystal structure

It occurs in three modifications (α-Si ₃ N ₄ , β-Si ₃ N ₄ and γ-Si ₃ N ₄ ), which differ in their crystal structure .

• Nitrogen atoms are shown in blue, silicon atoms in gray
• 

  trigonal α-Si ₃ N ₄

• 

  hexagonal β-Si ₃ N ₄

• 

  cubic γ-Si ₃ N ₄

Technical silicon nitride is a non-oxide ceramic , which usually consists of β-silicon nitride crystals in a glassy solidified matrix. The glass phase portion reduces the hardness of silicon nitride compared to silicon carbide , but enables the columnar recrystallization of the β-silicon nitride crystals during the sintering process , which results in a significantly increased fracture toughness compared to silicon carbide and boron carbide .

Manufacturing

High quality Si ₃ N ₄ powders form the basis of the silicon nitride ceramics. These arise z. B. on the reaction of pure silicon with nitrogen at 1000 to 1400 ° C. The silicon is previously obtained from quartz sand (SiO ₂ ), usually by reduction in an electric arc at 2000 ° C. The Si ₃ N ₄ powder is then isostatically sintered in a gas pressure sintering furnace at a nitrogen (N ₂ ) overpressure of approx. 2000 bar . The high mechanical pressures are necessary in the manufacturing process in order to almost completely eliminate the residual porosity.

The qualitative end product is cubic silicon nitride with a density of up to 3.9 g / cm ³ , according to the already known α- and β-Si ₃ N ₄ -phases of tri- and hexagonal silicon nitride forms of ≈3.2 g / cm ³ .

The material properties of another orthorhombic δ-Si ₃ N ₄ phase of silicon nitride currently only exist in the model.

use

The high fracture toughness in combination with small defect sizes gives silicon nitride one of the highest strengths among the engineering ceramic materials. Due to the combination of high strength, low coefficient of thermal expansion and a relatively low modulus of elasticity , Si ₃ N ₄ ceramic is particularly suitable for components subject to thermal shock and is used, for example, as an indexable insert for cast iron materials (including interrupted cuts) or for handling aluminum melts . With a suitable choice of a refractory glass phase (for example through the addition of yttrium oxide ), silicon nitride ceramics are suitable for use temperatures of up to about 1300 ° C. As a material in engines, silicon nitride has so far not been able to establish itself despite great efforts in research and development in the last few decades. Silicon nitride is also used as a special material in bearing technology for hybrid bearings (rolling elements made of Si ₃ N ₄ ) and all-ceramic bearings (rolling elements and races made of Si ₃ N ₄ ).

A special application are measuring tips ( cantilevers ) with which samples are resolved down to the atomic level in atomic force microscopes . They are made from silicon wafers and their surface is made particularly resistant to mechanical wear by a silicon nitride layer.

Semiconductor technology

In semiconductor technology , silicon nitride is used as an insulation or passivation material in the manufacture of integrated circuits ; in charge trapping memories , the non-conductor serves as a storage layer for bound electrical charges. In addition, it is used as a masking and stopping material in many processes, for example in the local oxidation of silicon ( LOCOS process ) or chemical-mechanical polishing . The advantage here is a different chemical behavior towards etching agents compared to the standard material silicon dioxide , that is, etching agents are used that attack silicon dioxide but not silicon nitride, or vice versa. For example, silicon nitride is usually wet-chemically etched with hot 85 percent phosphoric acid. Furthermore, silicon nitride is used in photovoltaics as an anti-reflective and passivation layer . The efficiency of the solar cells or modules is increased by lower reflection losses.

The deposition of silicon nitride layers essentially takes place by means of two chemical vapor deposition (CVD) methods : low-pressure CVD ( LPCVD ) and plasma-assisted CVD (PECVD). The properties of these layers are usually not identical to those of "ceramic silicon nitride", so they are usually less hard. In addition, depending on the deposition method, tension in the applied silicon nitride layer can occur, u. a. because the lattice spacing and temperature coefficient of silicon and silicon nitride differ. In particular when using the PECVD technology, tension can be minimized by setting the process parameters.

synthesis

• Direct nitridation using reaction bonding :

  ${\displaystyle \mathrm {3\ Si+2\ N_{2}\longrightarrow \ Si_{3}N_{4}} }$

• Carbothermal nitriding:

  ${\displaystyle \mathrm {3\ SiO_{2}+6\ C+2\ N_{2}\longrightarrow \ Si_{3}N_{4}+6\ CO} }$

• Diimide synthesis:

  ${\displaystyle \mathrm {SiCl_{4}+6\ NH_{3}\longrightarrow \ Si(NH)_{2}+4\ NH_{4}Cl} }$

  ${\displaystyle \mathrm {3\ Si(NH)_{2}\longrightarrow \ Si_{3}N_{4}+2\ NH_{3}} }$

• Chemical vapor deposition (LPCVD or PECVD) in microtechnology:

  ${\displaystyle \mathrm {3\ SiH_{4}+4\ NH_{3}\longrightarrow \ Si_{3}N_{4}+12\ H_{2}} }$

Web links

• Breviary Technical Ceramics - Nitrides. In: keramverband.de. June 4, 2018, accessed October 1, 2019 . 
• Properties of silicon nitride (Si ₃ N ₄ ). In: technical-ceramics.3mdeutschland.de. 3M Technical Ceramics , accessed October 1, 2019 . 

Individual evidence

1. ↑ ^{a b c d} data sheet silicon nitride from Sigma-Aldrich , accessed on 2011-04-23 ( PDF ).
2. ↑ ^{a b} Data sheet 40379 silicon nitride from AlfaAesar, accessed on 2017-08-06 ( 40379 PDF )(JavaScript required) .
3. ↑ Malcolm Back, William D. Birch, Michel Blondieau and others: The New IMA List of Minerals - A Work in Progress - Updated: September 2019. (PDF 2672 kB) In: cnmnc.main.jp. IMA / CNMNC, Marco Pasero, September 2019, accessed October 1, 2019 .  
4. ↑ Localities for Nierite. In: mindat.org. Hudson Institute of Mineralogy, accessed October 1, 2019 .  
5. ^ Klaus-Dieter Linsmeier: Handbook of Technical Ceramics . 2nd, revised and expanded edition. Süddeutscher Verlag onpact, Munich 2010, ISBN 3-937889-97-3 , p. 16 ( de.slideshare.net [accessed October 1, 2019]).
6. ↑ Wolfgang Kollenberg: Technical ceramics: Fundamentals-materials-process engineering . Vulkan, Essen 2004, ISBN 3-8027-2927-7 .
7. ↑ ^{a b} Chen Dong: Investigations of high-pressure and high-temperature behaviors of the newly-discovered willemite-II and post-phenacite silicon nitrides . In: Chinese Physics B . tape 22 , no. December 12 , 2013, p. 1–6 , doi : 10.1088 / 1674-1056 / 22/12/126301 ( cpb.iphy.ac.cn [PDF; 307 kB ; accessed on October 1, 2019]).
8. ↑ Alexander A. Gromov, Liudmila N. Chukhlomina: Nitride Ceramics . Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33755-2 , pp. 196 (English, limited preview in Google Book Search).
9. ↑ Deposition of silicon nitride layers Si ₃ N ₄ . In: crystec.com. Crystec Technology Trading GmbH, December 18, 2018, accessed October 1, 2019 .