Liner is a thin conductive film deposited inside a via or trench after the barrier — providing adhesion between the barrier metal and the copper fill, promoting good wetting for electroplating, and enhancing electromigration resistance.

What Is a Liner?

• Material: Tantalum (Ta, BCC α-phase preferred for Cu adhesion), Cobalt (Co), or Ruthenium (Ru).
• Function:
• Adhesion: Cu does not stick well to TaN. The Ta liner provides the "glue."
• Wetting: Promotes uniform Cu seed deposition and void-free electroplating fill.
• EM Resistance: A strong Cu/liner interface resists electromigration mass transport.
• Thickness: 1-3 nm (scaled aggressively at advanced nodes).

Why It Matters

• Void-Free Fill: Without a proper liner, Cu electroplating produces voids and seams that cause open failures.
• Reliability: The Cu/liner interface is the critical path for electromigration lifetime. A weak interface = early failure.
• New Materials: Co and Ru liners at 7nm and below improve fill and EM but add process complexity.

Liner is the adhesive layer for copper wires — ensuring the metal fills cleanly and stays firmly bonded for the lifetime of the chip.