Dichlorosilane

Dichlorosilane

Stereo, skeletal formula of dichlorosilane with some explicit hydrogens added
Ball and stick model of dichlorosilane Spacefill model of dichlorosilane
Names
IUPAC name Dichlorosilane
Other names Silicic dichloride dihydride
Identifiers
CAS Number	4109-96-0 Y
Abbreviations	DCS
ChemSpider	55266 Y
EC Number	223-888-3
Jmol 3D model	Interactive image
MeSH	dichlorosilane
PubChem	61330
RTECS number	VV3050000
UN number	2189
InChI InChI=1S/Cl2H2Si/c1-3-2/h3H2 Y Key: MROCJMGDEKINLD-UHFFFAOYSA-N Y
SMILES Cl[SiH2]Cl
Properties
Chemical formula	SiH 2Cl 2
Molar mass	101.007 g mol−1
Appearance	Colourless gas
Density	1.22 g cm−3
Melting point	−122 °C (−188 °F; 151 K)
Boiling point	8 °C; 46 °F; 281 K at 101 kPa
Solubility in water	Reacts
Vapor pressure	167.2 kPa (at 20 °C)
Thermochemistry
Std molar entropy (So298)	286.72 J K−1 mol−1
Std enthalpy of formation (ΔfHo298)	−320.49 kJ mol−1
Vapor pressure	{{{value}}}
Related compounds
Related dichlorosilanes	Trichlorosilane Silicon tetrachloride
Related compounds	Dichloromethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Dichlorosilane, or DCS as it is commonly known, is a chemical compound with the formula H₂SiCl₂. In its major use, it is mixed with ammonia (NH₃) in LPCVD chambers to grow silicon nitride in semiconductor processing. A higher concentration of DCS:NH₃ (i.e. 16:1), usually results in lower stress nitride films.

It is an intermediate between silane and silicon tetrachloride.

Physical properties

Graphs obtained show different properties of dichlorosilane. The vapor pressure vs. temperature graphs shows that as temperature increases, vapor pressure also increases. The gas vs. heat capacity graph shows that as temperature increases, gas heat capacity increases until it hits .24 Cal/GRx°C. The enthalpy of vaporization decreases as temperature decreases, until it reaches 0 at its critical point at 180 °C. Liquid heat capacity increases as temperature increases, until the critical point is reached. Liquid density decreases as temperature increases, until the critical point is reached. Surface tension decreases steadily as temperature increases, until the surface tension is at 0 when the critical point is reached. Liquid viscosity decreases as temperature increases, until the critical point is reached. Gas thermal conductivity increases as temperature increases. Liquid thermal conductivity decreases as temperature increases, until the critical point is reached. Enthalpy of formation decreases at first as temperature increases, but then it begins to increase around 750 °C. Gibbs energy of formation increases linearly as temperature increases.

History

Dichlorosilane was originally prepared in 1919 by the gas-phase reaction of monosilane, SiH₄, with hydrogen chloride, 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, H₂SiO. Prosiloxane polymerizes rapidly in the liquid phase and slowly in the gas phase, which results in liquid and solid polysiloxanes [H₂SiO]_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 H₂SiCl₂ in benzene by brief contact with water, and the molecular weight was determined to be consistent with an average composition of [H₂SiO]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, [HSi(OH)O]_n, was produced. There was limited availability of dichlorosilane until the silicone industry grew.

Reactions and formation

• Trichlorosilane disproportionation is the preferable route of formation

2 SiHCl₃ ⇔ SiCl₄ + SiH₂Cl₂

• The waste gas that is formed from silicon production by the use of silicon tetrachloride or trichlorosilane, HSiCl₃, can be reduced with oxygen to form dichlorosilane.

Hydrolysis

Stock and Somieski completed the hydrolysis of dichlorosilane by putting the solution of H₂SiCl₂ in benzene in brief contact with a large excess of water. A large-scale hydrolysis was done in a mixed ether/alkane solvent system at 0 °C, which gave a mixture of volatile and nonvolatile [H₂SiO]_n. Fischer and Kiegsmann attempted the hydrolysis of dichlorosilane in hexane, using NiCl₂⋅6H₂O as the water source, but the system failed. They did, however, complete the hydrolysis using dilute Et₂O/CCl₄ 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 [H₂SiO]_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. 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.

External links

• Safety data sheet for dichlorosilane from Praxair®