Atomic layer deposition (ALD)
Beginner
A way of building a film one layer of atoms at a time, so it can be made extremely thin and even.
Novice
Two chemicals are pulsed over the wafer in turn. Each reacts only with the surface until the surface is used up, so each cycle adds a fixed sliver, about a tenth of a nanometer.
Expert
Self-limiting surface reactions give thickness control by cycle count and near-perfect conformality in deep, narrow features. Used for high-κ gate dielectrics, work-function metals and the inner-spacer fill in nanosheet transistors.
Explained in Making them (Transistors).
See also: Deposition, High-k/metal gate (HKMG).