Atomic layer epitaxy

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Atomic layer epitaxy (ALE), now more generally called Atomic Layer Deposition, is a specialized form of epitaxy that typically deposit alternating monolayers of two elements onto a substrate. The crystal lattice structure achieved is thin, uniform, and aligned with the structure of the substrate. The reactants are brought to the substrate as alternating pulses with "dead" times in between. ALE makes use of the fact that the incoming material is bound strongly until all sites available for chemisorption are occupied. The dead times are used to flush the excess material.

Atomic layer epitaxy (ALE) or atomic layer deposition (ALD) is a technique mostly used in semiconductor fabrication to grow thin films of thickness of the atomic order.

This technique has been invented in 1977 by Dr. Tuomo Suntola in 1977, in the University of Helsinki in Finland. Dr. Suntola tried in fact to grow thin films of Zinc sulfide to fabricate electroluminescent flat panel displays. The main trick used for this technique is the use of a self limiting chemical reaction to control in a very accurate way the thickness of the film deposited.

Compared to basic chemical vapour deposition for example, chemical reactants are pulsed alternatively in a reacting chamber and then chemisorb on to the surface of the substrate in order to form the monolayer. This is very tricky because the reaction is very easy to set up and it doesn’t require that many restrictions over the reactants, allowing the use of a wide range of materials.

ALE introduces two complementary precursors (e.g. Al(CH₃)₃ and H₂O) alternatively into the reaction chamber. Typically, one of the precursors will adsorb onto the substrate surface, but cannot completely decompose without the second precursor. The precursor adsorbs until it saturates the surface and further growth cannot occur until the second precursor is introduced. Thus the film thickness is controlled by the number of precursor cycles rather than the deposition time as is the case for conventional CVD processes. In theory ALCVD allows for extremely precise control of film thickness and uniformity.