Etch Defect Naming Notching Microtrenching
An etch defect gets named by where on the cross-section it appears and what it looks like under a microscope, not by which chemistry caused it — notching, micro-trenching, faceting, and grass (also called black silicon) are the four names that cover almost every unwanted shape an etch can leave behind, and all four trace back to the same root cause: ions and electrons don't travel in the perfectly straight lines a recipe assumes. Learning these four names is learning to read a defect-review SEM image before reading a single process log.
A few things explain why each of these four effects earned a distinct name instead of being lumped together as "etch non-idealities":
Notching is named for its location, right at a buried interface, because that's the only place the mechanism can occur. On an SOI wafer, etching through a conductive or semiconducting film down to a buried oxide lets charge build up on the insulator once the etch front arrives there; that local charge deflects incoming ions sideways just above the interface, carving a lateral notch exactly at the film-to-oxide boundary. The name is inseparable from the location because the physics — electron shading plus local charging — only turns on once the etch front reaches that specific buried surface.
Micro-trenching is named for appearing at the base of a sidewall, because it comes from ions bouncing off that same sidewall. Ions arriving at a steep angle near a sloped or vertical sidewall reflect off it like light off a mirror and land at the foot of the feature, adding extra etch rate in a narrow stripe right where the sidewall meets the floor. The name distinguishes it from ordinary overetch precisely because it's a geometric reflection effect tied to sidewall angle, not a uniform over-etch of the whole floor.
Faceting is named for the shape it leaves, a flattened corner, because the mechanism is angle-dependent sputtering, not chemical etching at all. Ion sputter yield peaks at a glancing angle rather than straight-on incidence, so a sharp mask corner — which presents many angles to the incoming ion flux — erodes faster than the flat top or sidewall next to it, rounding the corner into a facet. It shows up most on hard masks and metal masks that rely on physical sputtering resistance rather than chemical selectivity, which is why faceting is discussed as a mask-engineering problem as much as an etch-recipe one.
Grass, or black silicon, is named for its visual texture because the underlying cause is contamination acting as an accidental mask. Redeposited etch byproduct, metallic contamination, or particles too small to see with the naked eye can each locally shield the floor from the etch just long enough for a forest of needle-like silicon columns to grow underneath them — the same micromasking principle deliberately exploited to manufacture textured black-silicon surfaces for solar cells, but unwanted and yield-killing when it appears inside a device feature.
Read a defect-review image by these four names rather than by a generic "etch anomaly" label, and the root-cause search narrows immediately: notching points straight at charging on a buried interface, micro-trenching points at sidewall angle and ion reflection, faceting points at the mask material's sputter resistance, and grass points at contamination control — the name itself is the first diagnostic step, before a single additional measurement is taken.