Disilane – Present production methods

In Japan there have been several process developments of disilane, but only one producer is presently in the market. Japanese producer of monosilane Mitsui also produces disilane and trisilane as a byproduct of their process. Thus, their disilane production capacity is dependent upon their production of (mono-)silane. This is presently the sole large scale producer in the world. Disilane may be produced from acidic reactions with Mg2Si:

 

2 Mg2Si + 8HCl -> 4 MgCl2 + Si2H6 + H2

 

Dilute hydrochloric acid (5%) can be used. The chemical yield of disilane compared to monosilane increases with increase in temperature. To operate the reaction at 100oC, dilute sulfuric or phosphoric acid can be used. Mitsui employs the transformation of magnesium silicide with addition of ammonium chloride:

 

2 Mg2Si + 8 NH4Cl -> 4 MgCl2 + Si2H6 + 8 NH3 + H2

 

These processes require silicide of the alkaline earth. Another way of producing disilane is to start with monosilane and activate these molecules so that they can react. Ways of activation are by electric discharge and heating.

 

Another way believed to be less energy consuming and with simple process requirements is to use a catalyst. Japanese companies have filed patents on the subject, but they have not been implemented industrially. These patents use platinum group metal complexes or lanthanide complexes as catalysts.

 

Japanese company Showa Denko has filed a patent for producing trisilane and higher ones by disproportionation of mono- and disilanes. They claim the above mentioned methods all have many problems to be solved before commercialization; the most serious ones concerning the catalysts are too low activity and formation of bi-products. Concerning electric discharge, the method suffers from low yield, high energy consumption, and low selectivity for disilane compared to higher silanes according to Showa Denko. From the above it is clear that there are several methods that can be used to produce disilane. However, in addition to the yield and content of higher silanes, the chemical purity is imperative when it comes to use in photovoltaics and electronics. The concept of this project is therefore to start with sufficient pure monosilane and produce disilane at an acceptable yield, separate it, and recycle the unreacted silane.

 

It is well known to the semiconductor industry that thin films made by disilane have superior quality to ones made by silane. A large number of published papers show various advantages. Published works report a 50% improvement in film thickness uniformity and 25% in surface roughness. In addition, improved reproducibility and a more homogenous topography of the Si-layer when using disilane compared to monosilane is shown. Maybe the best economical argument for substituting disilane for silane is the energy saving and the smaller loss of silicon due to more than 200 K lower decomposition temperature (typically 450 vs. 700 ᵒC). Disilane is thus a precursor for the rapid, low temperature deposition of epitaxial silicon and silicon-based dielectrics.

 

Another published work reports improved properties of thin film transistors produced on Corning glass substrates. Today’s price of disilane is, however, too high to allow extensive use. Only special applications like solar cells for space can afford this use. Primus.inter.pares AS is a part owner of Polysilane AS, a company aimed at finding new production methods for di- and other polysilanes. So far, Polysilane AS has one patent application pending for a new low cost method for producing disiilane from monosilane.

Arbeidsfelt