Wiw (within-Wafer Variation)

WIW (Within-Wafer Variation)
Overview

Within-wafer variation describes parameter differences between dies at different positions across a single wafer, primarily caused by radial process gradients in deposition, etch, CMP, and lithography.

Common WIW Patterns

• Center-to-Edge: Most common pattern. Many processes have radial gradients (higher deposition rate at center, higher etch rate at edge, or vice versa).
• Bull's Eye: Concentric ring pattern from rotating wafer processes.
• Asymmetric: Gas flow direction or chamber geometry creates non-radial gradients.

Sources by Process

• CVD/PVD: Film thickness varies ±1-3% center-to-edge due to gas flow, temperature, and plasma density profiles.
• Etch: Rate varies with plasma density distribution and gas flow. Edge exclusion zone (1-3mm) has highest variation.
• CMP: Pad pressure profile creates center-fast or edge-fast removal patterns. Multi-zone carrier heads compensate.
• Lithography: Focus and dose variation across the wafer (lens field curvature, wafer flatness).
• Implant: Beam scan uniformity creates dose variation. Typically < 1% for modern implanters.

Metrics

• WIWNU (Within-Wafer Non-Uniformity): (σ / mean) × 100%. Targets: < 1-2% for film thickness, < 2-3% for etch CD.
• Range: Max - Min across all measurement sites.
• 49-point or 13-point measurement maps are standard.

Mitigation

• Multi-zone process control (separate heaters, gas injectors, or pressure zones for center vs. edge).
• APC (Advanced Process Control): Feed-forward/feedback correction of recipe parameters based on incoming wafer measurements.
• Edge ring optimization (etch): Tune edge ring height and material to match edge plasma conditions to center.