Wafer-to-Wafer (W2W) Control is a run-to-run control strategy that adjusts process parameters between individual wafers — providing finer control granularity than lot-to-lot R2R control by accounting for within-lot variability such as slot position effects.

How Does W2W Control Work?

• Per-Wafer Measurement: Measure the critical output for each wafer (not just lot averages).
• Per-Wafer Update: Apply EWMA or model-based correction to adjust the recipe for the next wafer.
• Slot-Dependent Effects: Compensate for known slot-to-slot variations in batch processes (furnace position effects).
• Threading: Controller state is maintained per-chamber for multi-chamber tools.

Why It Matters

• Within-Lot Uniformity: Reduces wafer-to-wafer variation within a lot (not addressed by lot-to-lot R2R).
• Single-Wafer Tools: Natural control granularity for single-wafer process tools (etch, CVD, PVD).
• Tighter Specs: Advanced nodes require tighter within-lot variation, making W2W control increasingly necessary.

W2W Control is individual wafer tuning — adjusting the recipe for each wafer instead of each lot for tighter process control.