Etch Reentrant Profile Budget
# Etch Reentrant Profile Budget
An etch reentrant profile budget defines how much a feature's opening may narrow toward its top before the next fill step can no longer fill it without a void, and the evidence needed to measure that narrowing and test it against the real deposition. It links the etched profile to fillability rather than treating the two as separate problems.
A reentrant profile is one whose opening is narrower near the top than deeper inside, often from an overhang at the mouth or a mid-depth bulge. This budget does not restate the straight-wall angle calculation or the bowing phenomenon, which are covered separately; it concentrates on the fill consequence of a top-narrowing shape.
## See why the mouth matters most
A conformal or near-conformal deposition adds material on every surface, including the top corners of the opening. If the opening already narrows at the top, the growing film can close the mouth before the lower part of the feature is filled, sealing a void inside.
The sketch shows an overhang at the mouth and a void trapped near the base. It is not a deposition model and does not predict a void for any real process; whether a void forms depends on the specific profile and the fill step together.
Reported integration work supports the dependence. A gap-fill study notes that a high-aspect-ratio pattern with an ideal positive slope can be filled, while a real profile with a negative slope, taper or bowing is harder to fill. A trench-refill study achieved a keyhole-free result by producing a positive taper near vertical, adjusting it by lowering the etch-step time. Both treat the profile as the thing that must be made compatible with the fill.
## Measure the limiting opening, not the average
A single width or a nominal angle does not capture the risk. Find the narrowest point of the opening and record where along the depth it occurs, because that location is where a fill is most likely to pinch off.
Report the profile sign explicitly: positive-tapered (wider at top), vertical, or reentrant (narrower at top). A mean width can read as acceptable while a local overhang makes the feature unfillable by a particular deposition.
A bowing study of small holes found that below a certain diameter the profile became bowed regardless of etch rate, attributing it to deposition of etch products on the walls. That shows a reentrant or bowed shape can be a systematic outcome of the feature size and chemistry, not a random excursion.
## Build the budget around the fill step
The tolerable profile is defined by the deposition that will fill it. A highly conformal atomic-layer process, a flowable fill and a sputter-assisted high-density process each tolerate different amounts of top narrowing, so a profile acceptable for one can fail another.
Reported process work treats this as a joint window. An STI gap-fill study where the trench angle exceeded a threshold found one deposition could not fill it void-free and used an alternating deposition-and-etch approach, re-opening overhangs between deposition steps. The etch and fill were engineered together, not separately.
State the aspect ratio, the limiting opening and the fill chemistry together. A profile budget written without naming the fill step cannot be checked, because fillability is a property of the pair.
## Act on the cause and keep the side effects visible
If a profile is reentrant, options include adjusting the etch to a positive taper, changing the fill to a more conformal or flowable process, or interleaving a re-opening etch during fill. Each has costs: etch changes affect CD, selectivity and mask survival; fill changes affect throughput, stress and material; interleaved etch adds steps and can remove intended material.
No single remedy is universal. The requirement comes from the device and the chosen fill, not from a rule that every profile must be vertical or that every overhang is fatal.
## Close the budget with traceable evidence
Specify the limiting opening, its location, the profile sign, the aspect ratio, the fill step and the void-free criterion, at a named checkpoint. Retain recipe revision and chamber history so a later profile can be compared with the earlier one.
A reentrant profile budget measures the limiting opening and tests it against the actual fill step. Profile measurement, fill compatibility and acceptance are separate questions, and each needs its own evidence.
## Sources and scope
- [Process Window Engineering for Void-Free STI Gap Filling Using Integrated PEALD and Virtual DOE (AVS ALD)](https://www.avsconferences.org/ALD2026/Sessions/SupplementalDocumentDownload/88139?sessionId=88479): reports a trench angle beyond a threshold that one deposition could not fill void-free, and an alternating deposition-and-etch approach that re-opens overhangs.
- [Mechanism producing bowed profiles in the etching of low-k organic films (ECS)](https://www.electrochem.org/dl/ma/201/pdfs/0431.pdf): finds that holes below a certain diameter become bowed regardless of etch rate, attributed to deposition of etch products on the sidewalls.
Original diagrams and explanatory synthesis. No universal profile limit, fill-compatibility rule or qualified recipe is implied.