what are high aspect ratio features and why do they challenge deposition

ASPECT RATIO = depth ÷ width of a feature (a trench or via). A HIGH-ASPECT-RATIO feature is DEEP and NARROW — like a tall, skinny well. WHY IT CHALLENGES DEPOSITION: you have to coat the BOTTOM and SIDEWALLS of a deep narrow hole evenly — the opening can 'pinch off' at the top (closing the mouth before the bottom fills), leaving a VOID in the middle/bottom. This is why: 1) PVD (line-of-sight) can't reach the deep bottom well — poor step coverage. 2) You need CVD/ALD (Lesson 52) for conformal coverage. 3) Filling it void-free (like copper in a via) needs the right liner + fill process. The one-line frame: 'A high-aspect-ratio feature is a deep narrow well — and the challenge is coating the bottom without pinching the top shut, which is why you need conformal ALD and careful fill.' This directly explains why metal-deposition tools exist.