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.