Within-Wafer Non-Uniformity (WIWNU) measures thickness variation across a single wafer after CMP, critical for maintaining electrical specifications. Definition: WIWNU = (standard deviation of thickness measurements) / (mean thickness) x 100%. Typically reported as percentage. Target: <3% for most CMP processes. Advanced nodes target <1% for critical layers. Measurement: Film thickness measured at multiple points across wafer (49 or more sites). Edge exclusion zone typically 3-5mm. Sources of non-uniformity: Pad pressure distribution (center vs edge), slurry flow and distribution, wafer carrier design, retaining ring wear. Center-fast vs edge-fast: Common CMP non-uniformity signatures. Center of wafer polishes faster or slower than edge. Pressure zones: Modern CMP carriers have multiple pressure zones (3-7 zones) allowing independent control of removal rate across wafer radius. Retaining ring: Ring around wafer conditions pad near wafer edge, affecting edge uniformity. Retaining ring pressure is a key tuning parameter. Profile control: Combination of zone pressures, retaining ring pressure, pad conditioning, and slurry flow tuned for flat post-CMP profile. Incoming variation: Non-uniform incoming film thickness (from CVD or PVD) adds to CMP uniformity challenge. SPC monitoring: WIWNU tracked as key process control metric. Drift triggers corrective action.