ALD belongs to the CVD methods. Opposite to other CVD methods, you have a cyclic flow of materials into the reaction chamber, separated by an inert gas to separate two or more different reactions. The different part reactions are self-restricting, i.e. the basic materials don't react which themselves, leading to maximum one monolayer being formed per sub-reaction step.
The self-restriction is the characteristic of the ALD process. This has certain advantages. ALD allows a very high precision in controlling layer thickness and, furthermore, a very conformal deposition. The disadvantage is the typically very low deposition speed (a monolayer per cycle). New designs in ALD reactors, in which the different gasses are specially separated and the substrate is traveling through different chambers, can speed up the process. Hope that helps.
thanks a lot
I think ,Similar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction
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