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Process

The process of Silicon Epitaxy is essentially a CVD* process used for depositing thin films of single-crystal silicon on single crystal silicon substrate and it is used extensively in the microelectronic and semiconductor industries.
The requirements of the industry from this process are highly demanding, i.e., epitaxial silicon films must have an excellent thickness uniformity and excellent quality (minimum defects in the epitaxial layer).
The growth of epitaxial silicon is usually performed by means of thermally activated process, where the substrates are held over a heated susceptor placed in a quartz or a stainless-steel chamber under atmospheric or reduced pressure conditions.
Quartz-wall reactors (popularly known as cold-wall reactors), whose external walls are continuously cooled by recircultating air or water, are, however, more popularly used.
These reactors are characterized by a very high temperature difference between the susceptor and the reactor external walls.
The reactant feed to these reactors usually contains a deposition precursor (compounds of Si, H, Cl) highly diluted in a carrier gas (H, N, etc.) and a controlled amount of a dopant compound. The temperature at which the deposition process is performed and the sensitivity of the deposition process to oxidizing impurities are dependent on the hydrogen content in the precursor, i.e., the lower the hydrogen content, the higher the deposition temperature and the lower the process sensitivity to oxidizing impurities.

A schematic representation of the silicon Epitaxy Process is shown in the figure:
In general, the several steps that occur in a CVD process are as follows:
  • transport of gaseous species toward the growing surface in a nonisothermal flow field;
  • surface processes, such as the absorption of precursors, the surface diffusion of adatoms over the terrace, and their incorporation into step kinks or island clusters;
  • desorption of reactants and by-products from the deposition surface;
  • and, finally, transport of these species back into bulk gas phase.
The temperature dependence of film growth rate has a general behaviour and this behaviour is established depending on which of the above steps are the rate-determining step for epitaxial silicon deposition.
At low temperatures the reaction kinetics is usually the rate-limiting step, leading to growth rates that are strongly dependent on the temperature, but at higher temperatures transport is the rate-limiting step, leading to growth rates that are not very strongly dependent on temperature.
The transition temperature between one regime and the next is strongly dependent on the type of precursor used and, also, on the geometrical configuration of the considered reactor. [1]

Gases used in epi reactors :
    Silicon sources :
    • Dichlorosilane (DCS)
    • Trichlorosilane (TCS)
    • Silicon tetrachloride (TET)
    Dopants:
    • Arsine
    • Phosphine
    • Diborane
    Carrier:
    • H
    Purge:
    • N
    Chamber etch:
    • HCl
    [2]

Note:
* CVD = Chemical Vapor Deposition

References:
[1] M.Masi, S. Kommu, "Silicon Epitaxy", Epitaxial Growth Modeling
[2] V. Pozzetti, "Silicon Epitaxy", Epitaxial Growth Facilities, Equipment, and Supplies



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