R2R (Run-to-Run) control automatically adjusts process recipe parameters between consecutive runs based on metrology feedback to compensate for process drift and disturbances. Concept: After each run (lot or wafer), metrology result compared to target. Controller calculates recipe correction for next run. Continuous closed-loop optimization. EWMA controller: Most common algorithm. New recipe = weighted average of current correction and previous recipe. Lambda parameter controls responsiveness vs stability. Model-based: Process model (y = a*x + b) relates recipe parameter (x) to output metric (y). Controller inverts model to calculate required recipe change. Dead-band: Small deviations within dead-band ignored to prevent over-correction from measurement noise. Gain scheduling: Controller gain adjusted based on process state. More aggressive correction after large disturbances, conservative during stable operation. Applications: Litho dose adjustment based on post-develop CD. Etch time correction based on post-etch CD. CMP polish time based on incoming thickness. CVD time/temperature based on post-dep thickness. Thread-based: Separate controllers for different product/tool/chamber combinations (threads). Each thread tracks its own process state. Stability: Controller must be stable - over-correction causes oscillation. Under-correction allows drift. Tuning is critical. Qualification: New R2R controllers qualified by simulation and phased production deployment. Disturbance types: Drift (gradual), shift (sudden), lot-to-lot variation. Controller must handle all three. Integration: R2R controller interfaces with tool controller (SECS/GEM) and metrology database.