Dichlorosilane, or DCS as it is commonly known, is a chemical compound with the formula H
2SiCl
2. In its major use, it is mixed with
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
(NH
3) in
LPCVD
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
chambers to grow silicon nitride in semiconductor processing. A higher concentration of DCS·NH
3 (i.e. 16:1), usually results in lower
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
nitride films.
History
Dichlorosilane was originally prepared in 1919 by the gas-phase reaction of
monosilane
Silane is an inorganic compound with chemical formula, . It is a colourless, pyrophoric, toxic gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Sila ...
, SiH
4, with
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
, HCl, and then reported by Stock and Somieski. It was found that in the gas phase, dichlorosilane will react with water vapor to give a gaseous monomeric
prosiloxane, H
2SiO. Prosiloxane polymerizes rapidly in the liquid phase and slowly in the gas phase, which results in liquid and solid polysiloxanes
2SiOsub>n. The liquid portion of the product, which is collected via vacuum distillation, becomes viscous and gelled at room temperature. Hydrolysis was done on a solution of H
2SiCl
2 in benzene by brief contact with water, and the molecular weight was determined to be consistent with an average composition of
2SiOsub>6. Through analytical and molecular weight determinations, n was decided to be between 6 and 7. Then, through more experimentation with the product, it was determined that n increases as time increases. After being in contact with the aqueous hydrolysis medium for a longer period of time, a polymer,
Si(OH)Osub>n, was produced. There was limited availability of dichlorosilane until the silicone industry grew.
[Seyferth, D., Prud'Homme, C., Wiseman, G., Cyclic Polysiloxanes from the Hydrolysis of Dichlorosilane, Inorganic Chemistry, 22, 2163-2167]
Reactions and formation
Most dichlorosilane results as a byproduct of the reaction of HCl with silicon, a reaction intended to give
trichlorosilane
Trichlorosilane is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to pr ...
.
Disproportionation
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...
of trichlorosilane is the preferred route.
[Vorotyntsev, V., Mochalov, G., Kolotilova, M., Kinetics of Dichlorosilane Separation from a Mixture of Chlorosilanes by Distillation Using a Regular Packing, Theoretical Foundations of Chemical Engineering, 38(4), 355-359]
:2 SiHCl
3 SiCl
4 + SiH
2Cl
2
Hydrolysis
Stock and Somieski completed the hydrolysis of dichlorosilane by putting the solution of H
2SiCl
2 in benzene in brief contact with a large excess of water.
[Seyferth D., Prud’Homme C., Linear Polysiloxanes from Dichlorosilane, Inorganic Chemistry, 23, 4412-4417] A large-scale hydrolysis was done in a mixed ether/alkane solvent system at 0 °C, which gave a mixture of volatile and nonvolatile
2SiOsub>n. Fischer and Kiegsmann attempted the hydrolysis of dichlorosilane in hexane, using NiCl
2⋅6H
2O as the water source, but the system failed.
They did, however, complete the hydrolysis using dilute Et
2O/CCl
4 at -10 °C. The purpose of completing the hydrolysis of dichlorosilane is to collect the concentrated hydrolysis products, distill the solution, and retrieve a solution of
2SiOsub>n oligomers in dichloromethane.
These methods were used to obtain cyclic polysiloxanes.
Another purpose for hydrolyzing dichlorosilane is to obtain linear polysiloxanes, and can be done by many different complex methods.
The hydrolysis of dichlorosilane in diethyl ether, dichloromethane, or pentane gives cyclic and linear polysiloxanes.
Decomposition
Su and Schlegal studied the decomposition of dichlorosilane using transition state theory (TST) using calculations at the G2 level. Wittbrodt and Schlegel worked with these calculations and improved them using the QCISD(T) method.
[Walch, S., Dateo, C., Thermal Decomposition Pathways and Rates for Silane, Chlorosilane, Dichlorosilance, and Trichlorosilane, Journal of Physical Chemistry, 105, 2015-2022] The primary decomposition products were determined by this method to be SiCl2 and SiClH.
Ultrapurification
Dichlorosilane must be ultrapurified and concentrated in order to be used for the manufacturing of semiconducting
epitaxial silicon layers, which are used for microelectronics. The buildup of the silicon layers produces thick epitaxial layers, which creates a strong structure.
Advantage of use
Dichlorosilane is used as a starting material for semiconducting silicon layers found in microelectronics. It is used because it decomposes at a lower temperature and has a higher growth rate of silicon crystals.
Safety hazards
It is a chemically active gas, which will readily hydrolyze and self ignite in air. Dichlorosilane is also very toxic, and preventative measures must be used for any experiment involving the use of the chemical. Safety hazards also includes skin and eye irritation and inhalation.
[Praxair Material Safety Data Sheet (2007)]
References
{{Reflist
External links
Safety data sheetfor dichlorosilane from Praxair®
Chlorosilanes
Inorganic silicon compounds