Atomic Layer Deposition yields excellent step coverage and builds a film one atomic layer at a time. Unlike LPCVD or PECVD, ALD does not mix precursors but alternates them in pulses; the self-limiting surface reaction makes for a simple process set-up. For aluminium oxide, TriMethylAluminium (TMA) reacts with H2O to form Al2O3: one TMA pulse saturates the surface, excess is purged, one H2O pulse produces one atomic layer of Al2O3, and repeating the pulses gives the desired thickness — roughly 1 Å per cycle.