Feed-Forward Control

Feed-Forward Control is a process control strategy that uses upstream measurements to adjust downstream process parameters — compensating for known incoming variations before they cause downstream defects, rather than correcting after measuring the output.

How Does Feed-Forward Control Work?

• Measure Upstream: Measure a parameter at process step $N$ (e.g., film thickness after deposition).
• Predict Impact: Use a process model to calculate how the measured variation will affect step $N+1$.
• Adjust: Modify step $N+1$ parameters to compensate (e.g., adjust etch time if film is thicker than target).
• Result: The output of step $N+1$ is closer to target despite incoming variation.

Why It Matters

• Proactive: Corrects for known disturbances before they affect the process (unlike feedback, which waits for errors).
• Litho-Etch: Classic application: feed-forward CD correction from post-litho measurement to etch recipe.
• Stacking: Can chain multiple feed-forward stages through the process flow.

Feed-Forward Control is planning ahead in manufacturing — using upstream measurements to pre-compensate downstream processes before errors occur.