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Also called ALE
(Atomic Layer Epitaxy), Atomic Layer Depostion (ALD)
is a deposition method that was introduced by Dr.
Suntola in 1974 [1], and has the
capability of controlling the deposition thickness
of thin films to the order of one atomic monolayer.
Chemical gases (precursors) are carefully selected
for ALD so that after a single layer of the gas
sticks to the surface, the surface is passivated.
Before the next layer is deposited the passivating
atoms must be removed in various ways (chemical
reactions, thermal spikes, etc).
An example of
silicon ALD using thermal spikes as reported by the
Nishizawa group[2]
Nishizawa et al.
use disilane (Si2H6)as the precursor
gas. At 450oC,
disilane dissociatively adsorbs into two SiH3
groups that stick to the silicon surface. At 450oC,
unstable hydrogen atoms desorb and form Si-H on the
surface. This hydrogen terminated surface is not
reactive so excess disilane molecules are not
chemisorbed to the surface. By spiking the temperature to 550oC,
the Si-H bonds are broken and bare silicon atoms
stay on the surface.
The bare silicon surface is chemically
reactive and another layer of precursor
molecules can now be
deposited. In this way, the layer thickness
is controlled by the number of repetitions of the ALD
cycle. |
