Etch Faceting Budget
# Etch Faceting Budget
An etch faceting budget defines how much corner rounding an integration can tolerate at a masking edge or a feature's top corner, and the evidence needed to measure and attribute it. It describes a sloped, beveled region that forms where an upper corner erodes faster than the surrounding flat surface, converting a sharp edge into an angled facet rather than leaving a loss along the sidewall body or at its base.
Faceting is a top-corner phenomenon driven by angular ion flux, distinct from the base-corner and sidewall-body effects covered by neighboring budgets. A mask or film corner exposed to ions arriving across a range of angles erodes unevenly, with the corner itself receiving a higher effective sputter yield than the adjacent flat field, and the eroded material redistributes nearby rather than disappearing cleanly.
## Locate the facet relative to the top corner
The facet appears specifically at the upper corner of a masking edge or a feature that stands above the surrounding field, where oblique ion incidence sputters the corner at a higher net rate than the flat top or the vertical sidewall receive individually. A rounded edge that forms later, deep in the trench, or along the sidewall body itself, describes a different geometry and belongs under a different term even when both are consequences of angular ion flux.
Common contributors include a wide angular spread in the incident ion distribution, low selectivity between the mask and the underlying film so that mask erosion directly transfers a changing corner angle downward, and redeposition of sputtered mask material onto nearby sidewalls. Which mechanism dominates depends on the mask composition, the plasma's angular distribution and the aspect ratio of the feature, not on the apparent severity of the facet alone.
## Separate faceting from neighboring corner and sidewall effects
| Observation | What it describes | What it does not establish |
|---|---|---|
| Faceting | Sloped erosion at a top mask or feature corner | A loss anywhere along the sidewall body or base |
| Footing | Unremoved material at the sidewall base corner | A loss at the top corner or along the sidewall body |
| Mask erosion | Progressive thinning and rounding of the mask itself | A facet confined to the underlying film's corner |
| Sidewall roughness | High-frequency texture along the sidewall surface | A smooth angled bevel at either corner |
These four terms describe distinct locations and distinct geometric signatures around the same general feature. A report that calls any corner change "faceting" without checking whether the rounding sits at the top corner, tracks the mask's own erosion, or instead reflects texture along the sidewall cannot be checked against a budget written for faceting specifically.
## Build the faceting budget around the critical dimension
Fix the reference corner and the film it belongs to before reporting a facet as present or absent, since a facet measured on the mask's own corner and one measured on the transferred film's corner answer different questions about final device geometry. Report the bevel angle and how far down the edge it extends as two separate numbers rather than one combined "corner loss," since a shallow wide facet and a steep narrow one can consume different amounts of final critical dimension at similar apparent severity in a cross-section image.
## Qualify the consequence, not only the presence
A facet confined entirely within a mask's sacrificial thickness, with none of it transferred into the underlying film, may leave the final feature unaffected, while a smaller facet that reaches the transferred edge can still shift the finished critical dimension. Evaluate the measured facet against the specific transfer step and the budget for that critical dimension before treating any nonzero bevel as a defect.
A faceting budget ties a measured top-corner bevel to a fixed reference corner and the specific transfer step it threatens, rather than to a visual impression of a rounded edge. Mechanism attribution, dimensional measurement and functional consequence remain separate questions that each need their own evidence.
As a sibling to footing (row 1150), undercut (row 1149) and mask erosion budget rows, this entry covers only the top-corner bevel; it does not restate base-corner, sidewall-body or mask-thinning content already assigned to those neighboring rows.
## Sources and scope
- Lam Research — Plasma Etch Fundamentals: process description of angular ion flux and sputter yield effects that influence corner erosion in plasma etch.
- Oxford Instruments — Etch Process Control: discussion of mask selectivity and redeposition effects near feature top corners.
Original diagrams and explanatory synthesis. No universal faceting limit, mechanism attribution or qualified process fix is implied.