Silicon Epitaxial Growth Process

In wafer fabrication, silicon epitaxial deposition, or epitaxy, refers 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.

Epitaxy is commonly done through chemical vapor deposition, which is basically a process that forms a 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.

For instance, the over-all reaction for the hydrogen reduction of SiCl4 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 and SiH2Cl2 are present during the silicon epitaxial growth. Based on the results of such studies, the following reactions are believed by experts to be taking place:

SiCl4 + H2 <--> SiHCl3 + HCl;

SiHCl3 + H2 <--> SiH2Cl2 + HCl;

SiH2Cl2 <--> SiCl2 + H2;

SiHCl3 <--> SiCl2 + HCl;

SiCl2 + H2 <--> Si + 2HCl;

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.

Reference:   Silicon Processing for the VLSI Era, Vol. 1: Process Technology