what is step coverage conformality on trenches vias

STEP COVERAGE / conformality = how well the film coats the SIDEWALLS and BOTTOM of a trench or via (a deep, narrow hole). Perfect conformality = same thickness everywhere including deep inside. WHY IT MATTERS: if a metal film doesn't fill the bottom and sidewalls of a via, you get a VOID — a gap in the conductor — which later becomes an OPEN (no connection) or a reliability failure. As features get deeper and narrower (higher aspect ratio), conformality gets harder, which is why you switch from PVD (poor step coverage) to CVD to ALD (best) for the critical fill steps. The one-line frame: 'Poor step coverage means voids, and voids mean opens — so filling deep features evenly is what makes a working interconnect.' This connects to why ALD/liners (Lesson 52, 57) exist.