alignment marks

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.