7.4.5 Atomic layer deposition (ALD)
For very thin conformal films, where rate is unimportant but precise thickness control is critical, a form of CVD allows deposition one monolayer at a time. One precursor gas is introduced into the chamber, which is then pumped away leaving only a monolayer adsorbed onto the wafer and chamber walls. The second precursor gas can then be supplied to complete the reaction at the surface, and then this gas is pumped away along with any gaseous reaction products. This cycle is repeated several times per second using fast-switching valves, to build up the film layer by layer.
ALD is used in dynamic random-access memory (DRAM) capacitor manufacture to deposit high dielectric constant insulators such as zirconia: a monolayer of adsorbed ZrCl4 is reacted with water vapour to form ZrO2 and volatile HCl. As these insulating layers are extremely thin (to increase the capacitance) they must be of very high quality, which justifies the use of such a slow deposition technique (less than 1 nm min−1).
Deposition temperatures for ALD are typically lower than CVD, to aid the adsorption, and the chamber walls may be kept hotter than the wafer, in contrast to conventional CVD. As with CVD batches of wafers can be processed together, which slightly mitigates the inherent slowness of the technique.