Via-First vs. Via-Last is the choice in dual-damascene BEOL integration of whether to pattern the via or the trench first — each approach has different advantages for alignment, etch, and fill at advanced interconnect dimensions.

Via-First

• Sequence: Pattern via → etch via into dielectric → pattern trench → etch trench (stopping partway).
• Advantage: Via is aligned to the layer below (better via-to-metal alignment).
• Challenge: Via hole must survive trench litho (resist fill, exposure over topography).

Via-Last (Trench-First)

• Sequence: Pattern trench → partial trench etch → pattern via → etch via through remaining dielectric.
• Advantage: Trench is patterned on a planar surface — better lithographic control.
• Challenge: Via must be aligned to the trench (via-to-trench overlay).

Why It Matters

• Overlay: The via-first/via-last choice affects which alignment is tighter — critical at small via sizes.
• Etch Control: Each approach has different etch challenges (via protection, depth control).
• Node-Specific: The optimal choice can change with each technology node.

Via-First vs. Via-Last is choosing the patterning order — a critical integration decision that affects alignment, etch, and reliability of interconnect vias.