why does ald give the best conformality

The WHY: ALD builds the film one atomic layer at a time through self-limiting surface reactions. In each cycle, the first precursor binds only to available surface sites, then a purge removes the excess, then the second precursor reacts with those sites — so the growth stops when the surface is saturated. Because it's surface-limited and you repeat it, the film grows at the SAME rate everywhere — on the flat top, the sidewalls, and the bottom of a deep trench. That's why ALD is the most conformal. The trade-off: it's SLOW (each layer is a cycle with purges), so you use it where conformity matters most — thin liners/barriers in deep features. The one-line frame: 'ALD is conformal because it's self-limiting — every surface gets one layer per cycle, so deep features coat evenly — the cost is speed.'