Etch time, overetch and selectivity calculator
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KEY POINTS
Film thickness and etch rate vary across the wafer, so stopping at the average time leaves film at the slow spots, and overetch of about 20 to 50% of the just-etch time is needed. Where the film clears early, the underlayer is attacked for the rest of the etch, so underlayer loss is set by selectivity and overetch time.
Inputs
Underlayer loss exceeds the allowed value. Consider switching to more selective conditions (a gas or bias change in the overetch step).
Etch rate is assumed constant over time, and nonuniformity is estimated for the worst case, with the thickest film where the rate is slowest. Real etches also see microloading from pattern density and aspect ratio dependent etching (ARDE), where the rate falls as aspect ratio rises.
Overetch, underlayer loss and remaining mask
| Overetch | Total time | Film removal | Maximum underlayer loss | Remaining mask |
|---|---|---|---|---|
| 0% | 120 s | Residue remains | 2.4 nm | 100 nm |
| 10% | 132 s | Fully cleared | 5.6 nm | 90 nm |
| 20% | 144 s | Fully cleared | 8.7 nm | 80 nm |
| 30% | 156 s | Fully cleared | 11.8 nm | 70 nm |
| 50% | 180 s | Fully cleared | 18.1 nm | 50 nm |
| 100% | 240 s | Fully cleared | 33.9 nm | 0 nm |
Why overetch is needed
Film thickness and etch rate always vary across the wafer, so stopping at the average time leaves film where it is thickest and the rate is slowest. Overetch prevents this; adding about 20 to 50% of the just-etch time is common. Overetch is also essential for removing film left along step sidewalls (stringers).
Where the film clears early, however, the underlayer is attacked for the entire overetch, so low selectivity to the underlayer means losing it. Real processes catch the moment the film clears with optical emission endpoint detection and then switch to more selective conditions for the overetch, avoiding both residue and underlayer loss.
FAQ
How much overetch is typical?
About 20 to 50% of the just-etch time. The minimum depends on thickness and rate nonuniformity, and stringers along steps may need more.
How do fabs limit underlayer loss?
Optical emission endpoint detection catches the moment the film clears, and the recipe then switches to more selective conditions for the overetch step.
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Last updated: October 23, 2026