etch selectivity budget

# Etch Selectivity Budget

An etch selectivity budget converts removal rates and exposure time into the thickness that a mask or protected layer can afford to lose. Selectivity alone is a ratio; a budget asks whether the full process can clear the target while preserving enough of every other exposed film.

## Start with the mask

Define target-to-mask selectivity as the target removal rate divided by the mask removal rate under the specified conditions. A larger value means less mask consumption for the same target-equivalent removal. Always state which material is the denominator.

Etch Selectivity Budgetthe mask must survive the complete removal windowStarting maskAfter etch50 nm30 nm lost20 nm left500 nm target + 20% added time at 20:1 selectivityMask loss = 600 / 20 = 30 nmCalculated constant-rate example · additional steps and variation need their own allowance.

For this calculated example, assume a 500 nm target film, constant target and mask rates, 20:1 target-to-mask selectivity, and a total duration 20% longer than the nominal target-clear time. The target-equivalent removal is 500 × 1.20 = 600 nm. The exposed mask loses 600 / 20 = 30 nm. Starting with 50 nm therefore leaves 20 nm in this model.

If the process requires at least 15 nm of residual mask, this simplified calculation leaves only 5 nm for other consumption or uncertainty. Breakthrough, BARC opening, thickness variation and later exposure are not automatically covered by the 20% added-time assumption. Allocate them explicitly rather than assuming the remaining 20 nm is all usable margin.

## Underlayer loss uses a different clock

A mask can be exposed during almost the whole etch. A buried stop layer begins direct exposure only after the target above it clears. Early-clearing and late-clearing features therefore have different protected-layer exposure times.

Underlayer Exposure Budgetthe protected layer starts losing thickness after it is exposedEarly-clear featureTarget remainsExposedLate-clear featureTarget remainsExposedTimeEtch endsEarlier clearing means a longer underlayer exposure.Schematic timing comparison · mask exposure and underlayer exposure are different budgets.

Suppose a protected film removes at 2 nm/min after exposure. A feature exposed for 0.5 min loses about 1 nm; one exposed for 1.5 min loses about 3 nm, assuming the same constant local rate. These values are a separate illustrative example, not measured performance or a recipe recommendation.

Do not estimate every underlayer loss by dividing the full initial target thickness by target-to-underlayer selectivity. Account for when that location actually becomes exposed. If a stop layer must subsequently be opened, treat that intentional removal as a separate step and budget the next landing material too.

## Build the filmstack budget

1. List the target, mask, stops and final landing in their physical order.
2. For each step, state which materials are exposed and for how long.
3. Estimate thickness loss using rates appropriate to the film, feature geometry and step conditions.
4. Include incoming thickness variation, delayed clearing, extra removal steps and the minimum acceptable residual thickness.
5. Validate clearing, profile and protected-layer loss together on representative patterned structures.

For a multistep model, total loss in one material is the sum of its loss in each exposure interval. If rates change significantly with time or local conditions, a constant-rate estimate must be replaced by a model or measurement that captures those changes. A blanket selectivity measurement does not establish patterned-feature selectivity, lateral mask retreat, charging damage or electrical integrity.

## Use the budget with the process flow

A gate-poly budget protects the gate dielectric during landing. An AlCu module must clear its metal and barrier stack while preserving the dielectric below. A spacer etchback balances horizontal clearing against liner loss, silicon recess and retained spacer width. Each module needs its own exposure history; one favorable ratio cannot describe all three.

The useful question is: after the last feature clears, how much protected material remains at the most exposed location? Check that result against the required geometry and electrical performance, not just the nominal etch rate.

## Sources and scope

  • [Applied Materials — Etch](https://www.appliedmaterials.com/sg/en/semiconductor/products/processes/etch.html): masking and pattern-transfer context.
  • [Applied Materials — Selective Etch](https://www.appliedmaterials.com/il/en/semiconductor/products/shape/selective-etch.html): selective removal and protected materials.
  • [Applied Materials — Silicon etch and soft landing](https://ir.appliedmaterials.com/news-releases/news-release-details/applied-materials-launches-silicon-etch-system-sub-015-micron): an example of changing conditions near a gate-oxide interface.

Original diagrams and arithmetic examples. Film dimensions and rates illustrate budgeting rather than a qualified manufacturing process.

Go deeper with CFSGPT

Get AI-powered deep-dives, save terms, and run advanced simulations — free account.

Create Free Account