APC (Advanced Process Control) uses real-time metrology feedback to automatically adjust process recipes, maintaining tighter process control than manual adjustments. Feedback control: Post-process metrology results used to adjust recipe for next lot. Example: if post-etch CD is 1nm above target, reduce litho dose for next lot. Feed-forward control: Pre-process measurements used to adjust current process. Example: incoming film thickness measured, etch time adjusted to compensate. R2R control: Run-to-Run controller calculates recipe adjustments between lots using EWMA (Exponentially Weighted Moving Average) or model-based algorithms. Control loop: Measure -> Compare to target -> Calculate correction -> Apply to recipe -> Measure again. Continuous optimization. Controlled parameters: Litho dose and focus, etch time and power, CMP pressure and time, CVD temperature and time, implant dose. Models: Linear or nonlinear models relate recipe parameters to output metrics. Models updated with ongoing data. EWMA: Exponentially Weighted Moving Average filters measurement noise while tracking process drift. Most common R2R algorithm. Multi-input multi-output (MIMO): Advanced APC controls multiple outputs simultaneously by adjusting multiple recipe parameters. Benefits: Tighter CD control, better uniformity, higher yield, reduced operator intervention, faster response to process drift. Integration: APC systems interface with tool controllers, metrology tools, and MES through SECS/GEM or EDA interfaces. Vendors: Onto Innovation (Angstrom), Rudolph/Onto, Applied Materials (iAPC), proprietary fab-developed systems.