timed etch

**Timed etch** is an etch control method where the process runs for a **predetermined, fixed duration** and then stops — regardless of the actual etch depth achieved. It is the simplest form of etch control and is used when the etch rate is well-characterized and stable. **How Timed Etch Works** - **Characterize**: Measure the etch rate (nm/min or Å/min) under specific process conditions through test wafers. - **Calculate**: Determine the required etch time based on: $\text{Time} = \frac{\text{Target Depth}}{\text{Etch Rate}} \times (1 + \text{Overetch \%})$ - **Run**: Execute the etch for exactly the calculated time. - **Verify**: Measure the actual etch result (depth, CD) to confirm it meets specifications. **When Timed Etch Is Used** - **Blanket Film Removal**: Etching uniform films without patterning where etch depth doesn't need to stop at a precise interface. - **Resist Descum**: Short, timed O₂ plasma treatment. - **Breakthrough Steps**: Removing thin, well-characterized barrier layers. - **Well-Controlled Processes**: When the etch rate is stable and reproducible (run-to-run variation < 2%). - **No Distinct Stop Layer**: When there's no material change to detect with endpoint methods. **Advantages** - **Simple**: No endpoint detection hardware or algorithms needed. - **Reproducible**: If the etch rate is stable, timed etch gives consistent results. - **Low Cost**: No additional metrology equipment integrated into the etch chamber. **Disadvantages** - **No Feedback**: If the etch rate drifts (due to chamber conditioning, consumable wear, or process drift), the timed etch doesn't compensate — it always runs the same duration. - **Chamber-to-Chamber Variation**: Different etch chambers may have slightly different etch rates — the same time gives different results. - **Film Thickness Variation**: If the incoming film is thicker on some wafers, a fixed etch time may under-etch them. - **No Overetch Protection**: Cannot determine when the target material is consumed — may over-etch or under-etch without knowing. **Safeguards** - **Regular Rate Monitoring**: Run monitor wafers periodically to check etch rate stability. - **APC (Advanced Process Control)**: Use measurements from previous wafers to adjust the etch time for the next wafer (feed-forward or feedback control). - **Built-In Margin**: Include sufficient overetch time to account for expected variation (typically 10–30% overetch). Timed etch is the **default approach** for non-critical etch steps and well-controlled processes — its simplicity makes it the workhorse of semiconductor etch operations when endpoint detection isn't necessary.