Epitaxial Growth Process
In wafer fabrication, silicon epitaxial deposition, or
to the process of growing a thin layer of single-crystal silicon over a
single-crystal silicon substrate. The growth of an epitaxial layer over
the substrate offers some advantages, including improvements in the
performance of bipolar devices, prevention of latch-up in CMOS circuits,
and improved doping control.
commonly done through chemical
vapor deposition, which is basically a process that
non-volatile solid film on a substrate from reactions of the appropriate
chemical vapors. There are four major chemical sources of silicon for
commercial epitaxial deposition: 1) silicon tetrachloride (SiCl4);
2) trichlorosilane (SiHCl3);
3) dichlorosilane (SiH2Cl2);
and 4) silane (SiH4).
Since a CVD process involves
chemical reactions, the use of chemical reaction equations is a good way
to describe the growth of epitaxial layers. In fact, each of the
chemical sources mentioned above may be described by an over-all
reaction equation that shows how the vapor phase reactants form the
silicon epitaxial film. For example, the over-all reaction for silicon
epitaxy by silane reaction may be written as follows: SiH4
--> Si + 2H2.
It must be pointed out, however, that such
equations for over-all reactions don't provide a complete picture of
what really transpires during the CVD process, especially with regard to
how the gas phase reactants really interact or how the epi species are
being adsorbed on the substrate surface.
the over-all reaction for the hydrogen reduction of
to form a silicon epitaxial
layer is as follows: SiCl4 + 2H2 --> Si + 4 HCl.
Yet, studies conducted by experts show that intermediate chemical
species such as SiHCl3
are present during the silicon epitaxial growth. Based on the results of
such studies, the following reactions are believed by experts to be
These equations confirm that
silicon epitaxy is not a simple process. Even if a given process may be
described by a single over-all reaction, the process in reality occurs
as a combination of many simultaneous chemical reactions.
The growth rate of an
epitaxial layer depends on several factors, which include: 1) the
chemical sources; 2) the deposition temperature; and 3) the mole
fraction of reactants.
Silicon Processing for the VLSI Era, Vol. 1: Process Technology
Copyright © 2005
All Rights Reserved.