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Etch Selectivity Calculator
Compare rate-based and consumption-based etch selectivity side by side, and estimate how much mask you need for a given etch depth. Enter your data below — everything is calculated in your browser.
① Rate ratio (blanket wafers)
S_rate
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② Consumption ratio (patterned wafer)
S_consumed
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③ Mask needed
Filled from ② if you entered it, otherwise from ①.
Mask needed ≥
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Calculated in your browser. Nothing is sent to a server.
How it is calculated
Rate-ratio selectivity (blanket wafers): S_rate = etch rate of the target film ÷ etch rate of the mask. Enter the two rates in the same unit.
Consumption-ratio selectivity (patterned wafers): S_consumed = depth etched in the target ÷ mask thickness consumed. This captures real process conditions that the rate ratio misses.
The mask estimate takes a target etch depth, an over-etch percentage, a selectivity value, and an optional margin in the same length unit. The formula is: mask needed ≥ target depth × (1 + over-etch fraction) / S + margin.
When both selectivities are available, the tool shows how much lower the consumption ratio is. Over-etch, ARDE (aspect-ratio-dependent etch), and mask faceting all pull the consumption ratio below the rate ratio.
Limits to keep in mind
- Using rate-ratio selectivity in the mask estimate gives a lower bound. The actual mask needed is often thicker, especially for high-aspect-ratio features where ARDE and faceting are significant. Prefer the consumption ratio when you have it.
- Selectivity is always a pair — name both films (e.g. target : mask or target : underlayer). Report which formula, which pair, and which feature you measured.
FAQ
- What is the difference between rate-ratio and consumption-ratio selectivity?
- Rate-ratio selectivity divides etch rates measured on blanket wafers. Consumption-ratio selectivity divides actual depths consumed on a patterned wafer. The consumption ratio is typically lower because it includes effects like ARDE and mask faceting.
- Why is my consumption-ratio selectivity lower than the rate ratio?
- Over-etch, ARDE, and mask faceting increase effective mask consumption on patterned wafers. These effects do not appear in blanket-wafer rate measurements.
- Which selectivity should I use for the mask estimate?
- Use the consumption ratio when you have it. The rate ratio gives a lower bound that often under-sizes the mask for high-aspect-ratio etches.
- Why do I need to specify over-etch?
- Over-etch is the extra time beyond endpoint to clear residual target material across the wafer. It adds to mask consumption. Enter it as a percentage of the main etch time.
Read the full article
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