Alignment marks are reference patterns on the wafer used to align each lithography layer to previous layers. Purpose: Scanner detects marks from prior layer to precisely position new exposure. Mark types: Cross, box-in-box, gratings. Different marks optimized for different detection methods. Placement: In scribe lines (between dies) and sometimes in die for intrafield measurement. Detection: Optical detection (laser scanning, imaging) measures mark position. Wafer alignment sequence: Global alignment (whole wafer), then die-by-die or field-by-field fine alignment. Mark degradation: Marks must survive all processing. Covered, etched, polished - must remain detectable. Zero layer: First lithography layer places alignment marks used by all subsequent layers. Hierarchy: Some marks for coarse alignment, others for fine. Multiple mark types per layer. Material contrast: Marks work through material contrast (oxide vs silicon, metal vs dielectric). Maintenance: Alignment mark quality monitored as process indicator.