Silicon tetrafluoride

• Silicon (IV ) fluoride
• Tetrafluorosilane

Colorless gas with a pungent odor

Gaseous

4.372 kg · m -3 ( 15 ° C)

-95.2 ° C

Readily soluble in water

Risk

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Silicon tetrafluoride ( SiF4 ), a silicon compound selected from the group consisting of silicon tetrahalides. It is a colorless, poisonous, non-flammable gas. In the semiconductor industry, silicon tetrafluoride is formed upon removal of silicon dioxide layers on wafers by means of hydrofluoric acid.

Production

Method 1

It is made of calcium fluoride and silica ( sand) with concentrated sulfuric acid. The following reactions are operating: implementation of fluorite to hydrogen fluoride

The hydrogen fluoride reacts with the SiO 2

Under excess acid hydrolysis of SiF4 is prevented.

However, the SiF4 thus obtained is only 80 - to 90 - percent. It contains oxygen- containing products such as F3Si -O- SiF3 or F3Si -O- SiF2 -O- SiF3. When transporting pipelines SiF4 in these molecules decompose into SiF4 and SiO2. Here, the SiO2 clogs the pipes. The chemical industry had the procedure of the transport of SiF4 developed in pipelines instead of the dangerous hydrogen fluoride to supply more distant production by pipeline with fluorides. Because of the problems, however, this method was not spread.

Method 2

When heating Bariumfluorosilikat SiF4 is formed, it can also be used by itself hexafluorosilicic:

Properties

Silicon tetrafluoride has a pungent odor and is strongly fuming when wet, the smoke is composed of hydrosilicofluoric and silica. It changes during cooling by Resublimation from the gaseous phase directly to the solid phase. The gas is liquefied at a pressure of 50 atm. Its critical temperature is -14.15 ° C, its critical pressure 37.2 bar, its triple point at -86.8 ° C and its triple point at 2,240 bar.