Feedback Control

Feedback Control is a process control strategy that uses downstream measurements (after processing) to adjust the process recipe for subsequent lots — correcting for systematic drift by comparing output measurements to targets and applying corrections to future runs.

How Does Feedback Control Work?

• Measure Output: Measure the critical parameter after processing (e.g., post-etch CD).
• Calculate Error: $e = \text{measured} - \text{target}$.
• Adjust Recipe: Modify the recipe for the next lot to reduce the error (e.g., change etch time).
• Controller: EWMA (Exponentially Weighted Moving Average), PID, or model-based controller determines the correction.

Why It Matters

• Drift Compensation: Automatically corrects for slow process drifts (chamber aging, gas line degradation).
• Standard Practice: Feedback R2R (run-to-run) control is implemented on nearly every critical process step.
• Combines with Feed-Forward: Most production uses combined feed-forward (inter-step) + feedback (intra-step) control.

Feedback Control is learning from the last lot — using post-process measurements to continuously improve the recipe for subsequent production.