CVD and PECVD are continuous processes: precursor flows in, the film grows, and thickness is set by deposition time. ALD, developed by Tuomo Suntola in the 1970s, splits the growth into two half-reactions that never happen at the same time. The two precursors are pulsed into the reactor alternately, separated by purges, and each of them reacts only with the surface left by the other. Each half-reaction stops on its own once the surface is saturated. Growth therefore proceeds in fixed increments, one cycle at a time, and the final thickness is simply the number of cycles multiplied by the growth per cycle.
The standard example is Al₂O₃ grown from trimethylaluminum (TMA) and water.
First pulse: TMA. The TMA vapor reacts with hydroxyl (–OH) groups on the surface. The aluminum binds to the surface oxygen and methane is released as a by-product. TMA does not react with itself, so once every accessible –OH site is occupied, adsorption stops. Longer exposure changes nothing. What remains is exactly one chemisorbed layer, on every surface the gas has reached.
First purge. Inert gas (N₂ or Ar) removes the unreacted TMA and the methane. Because of this purge, the two precursors never meet in the gas phase. This is where the purity of ALD films and their freedom from particles come from.
Second pulse: water. Water vapor reacts with the methyl groups of the adsorbed aluminum species, forming Al–O bonds and leaving behind a fresh hydroxyl-terminated surface, chemically identical to the starting surface but one atomic layer higher. This half-reaction is also self-limiting; once all methyl groups have been converted, it stops.
Second purge, and the cycle can start again.
Each cycle adds a fixed increment of material, typically 0.1 to 0.3 Å depending on the chemistry. The value is below a full monolayer because the ligands of the adsorbed precursor are bulky and block part of the surface sites. Repeating the cycle n times gives a film of n times the growth per cycle, which makes thickness control a matter of counting rather than timing.
Because saturation, not gas flux, determines how much material is deposited per cycle, every surface the gas can reach receives the same amount. Trenches, pores, aspect ratios beyond 100:1: as long as the pulse and purge times give the gas enough time to diffuse in and out, step coverage stays above 95%, where CVD and PVD fail completely. Thickness uniformity is better than ±1% over large substrates, and impurity levels stay below 0.1 at.% thanks to the separated half-reactions. Plasma-enhanced ALD replaces water with an oxygen plasma as co-reactant and brings the deposition temperature down toward room temperature, low enough for polymers. And since a recipe is just a sequence of pulses, nanolaminates and mixed oxides (Al₂O₃/TiO₂, HfSiO₄ and others) are obtained by interleaving cycles of different chemistries, with no change to the hardware.
In barrier multilayers, ALD oxide layers play the same role as PECVD SiO₂, decoupling defects and relieving stress between parylene interlayers, but they start from a much lower intrinsic defect density. A few tens of nanometers per oxide layer are sufficient, and the resulting stacks reach water vapor transmission rates on the order of 10⁻⁵ g/m²·day, several orders of magnitude below any polymer and roughly two orders of magnitude below PECVD-based multilayers.
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