ALD mode: precursor A gas diffuses into the trench and chemisorbs onto every bare site it reaches — including deep on the sidewalls and floor, since gas molecules randomly wander until they find an open bond, not just travel in straight lines. Once every reachable site is occupied the reaction is self-limiting: extra A molecules simply cannot stick anywhere and are swept away in the purge. Precursor B then pulses in and reacts only with the A-covered sites, completing one atomic layer everywhere at once — top, sidewalls and bottom alike — before its own purge. Because each half-reaction saturates uniformly, the film grows in exact, identical atomic increments regardless of depth, so conformality stays near 100%.
cycle = pulse(A → saturation) → purge
→ pulse(B reacts with A → 1 layer) → purge
thickness(depth) ≈ thickness(top) for all depths
PVD mode: sputtered atoms fly in straight, largely vertical lines from a source above the wafer (line-of-sight). A narrow, deep trench shadows its own lower sidewalls and floor — most atoms are captured by the top surface and the upper lip before they can travel far enough down to reach the bottom, so the top thickens quickly while the bottom stays almost bare.
- Cycle / flux speed — how fast pulses and purges (or the PVD flux) run.
- Saturation this pulse — fraction of eligible sites that have reacted during the current A or B pulse; watch it plateau near 100% and stop, even with gas still flowing — that plateau is the self-limiting step.
- Conformality — trench-bottom thickness divided by top thickness. ALD holds this near 100% at any aspect ratio; PVD collapses it as the trench gets deeper and narrower.