Liner Deposition is the thin film deposited on via and trench sidewalls and bottoms before filling with metal — providing adhesion, diffusion barrier, and nucleation functions that ensure reliable metal interconnect formation.

Why Liners Are Needed

• Copper diffuses rapidly through SiO2 and Si → kills transistors.
• Tungsten doesn't adhere to SiO2 directly → delamination.
• Liners provide: diffusion barrier (Cu), adhesion (W), nucleation surface for CVD/ELD.

Contact Liner (W Contacts)

Ti Adhesion Layer:

• PVD Ti, 5–20nm.
• Reacts with Si at contact bottom: Ti + Si → TiSi2 (lowers contact resistance).
• Provides adhesion for TiN above.

TiN Barrier Layer:

• CVD or PVD TiN, 10–30nm.
• Diffusion barrier: Prevents W from reacting with Si.
• Nucleation layer: CVD W nucleates uniformly on TiN (poor on SiO2).

Copper Via/Trench Liner (Dual Damascene)

TaN Diffusion Barrier:

• ALD or iPVD TaN, 2–4nm at advanced nodes.
• Excellent Cu diffusion barrier: Activation energy > 1.5 eV.
• Must be conformal in high-AR features (AR > 10:1).

Cu Seed Layer:

• PVD Cu, 10–50nm — nucleation layer for Cu electroplating.
• Must be continuous even at bottom corners — gap-fill challenge.
• At 5nm node: Seed may be replaced by fully-CVD or ALD Cu.

Scaling Challenge

• At 5nm node: TaN + Cu seed = 5–8nm of overhead in a 10nm-wide trench.
• Alternative barriers: Co, Ru metal barriers (< 2nm effective) — enable thinner liners.
• Ruthenium liner: Direct-plate without Cu seed, better resistivity, thinner possible.

Liner deposition is a critical integration challenge at each technology node — balancing barrier effectiveness with the overhead cost of film thickness becomes increasingly difficult as feature sizes approach single-digit nanometers.