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.