via-first dual damascene

**Via-First Dual Damascene** is a **patterning sequence where the via hole is etched first through the full dielectric stack, and then the trench is etched afterward** — partially into the top of the dielectric, using the via etch stop as a reference. **What Is Via-First?** - **Sequence**: Coat -> Expose via -> Etch via (full depth) -> Fill with sacrificial material -> Coat -> Expose trench -> Etch trench (partial depth) -> Remove sacrificial fill from via. - **Advantage**: Via is etched into a planar surface -> better lithographic focus and overlay. - **Challenge**: Must protect the via during trench etch (sacrificial fill or BARC). **Why It Matters** - **Overlay**: Better via-to-metal overlay because lithography is done on a flat surface. - **Preferred**: The dominant scheme at advanced nodes (14nm, 7nm, 5nm) for tighter overlay control. - **Complexity**: Requires additional processing steps (sacrificial fill/removal) but yields better results. **Via-First** is **drilling the hole before carving the channel** — prioritizing via placement accuracy, which is the tighter specification at advanced nodes.