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.