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Global loading effect: Thermal and
chemical loading effects in selective SiGe epitaxy
Rapid thermal reduced pressure chemical vapour deposition of
selective SiGe epitaxy is used for the base formation in the
fabrication of high performance heterojunction bipolar transistors.
This so-called loading effect is found to be due to two
significant effects, thermal and chemical. The thermal effect comes
from a differential heating, depending on the emissivity of the
substrate and on the deposition tool. This effect can induce a
variation in Si and SiGe growth rates when the growth is carried out in
the surface-kinetics limited regime, where the process is very
sensitive to the temperature. The chemical effect depends on the
polycrystalline silicon/total surface ratio and on the pad size.
One point is based on reduction of surface diffusion of
the adsorbed species on the oxide. In
non-selective epitaxy, this was achieved by introducing a thin silicon
polycrystalline seed layer on the oxide prior to
SiGe deposition. The thickness of this seed
layer had a crucial role on both the global and local loading effect,
and also on the epitaxial quality
Lower pressure can improve loading effect
different loading effects in selective epitaxial
deposition of silicon germanium on silicon (001) using different
silicon sources, such as silane or dichlorosilane, and other
conventional sources, such as germane, and hydrogen chloride in
hydrogen carrier gas, in a low-pressure chemical vapor deposition
system. Silane leads to lower relative deposition rates in a smaller
silicon area, while dichlorosilane shows the opposite trend. Flowing
silane and dichlorosilane simultaneously during deposition results in a
similar deposition rate independent of exposed silicon area.
Decreasing hydrogen chloride partial pressure is found to improve the loading
effect for both the silane- and dichlorosilane-based process for a
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