Etch Sidewall Profile Naming

An etch profile is named by comparing the sidewall angle to a single reference line — straight down — and the four names that result (vertical, tapered, bowed, re-entrant) describe the full vocabulary an etch engineer needs to diagnose a cross-section photo at a glance. Vertical means the sidewall tracks that reference line; tapered means it leans inward as it goes down (narrower at the bottom); bowed means it bulges outward in the middle before closing back up; re-entrant — also called undercut — means it leans the wrong way, wider at the bottom than the top. Every one of these names is a shorthand for a different failure mode or control target, not a cosmetic description.

A few things explain why these four names, and not some finer-grained set, became the industry's working vocabulary:

Vertical is the target, not a description of reality. No etch is perfectly vertical — the name marks the acceptable band around 90 degrees sidewall angle, typically within a degree or two depending on the node. Every other name in the vocabulary describes a specific direction of deviation from this unreachable ideal, which is why engineers always discuss profile in relation to vertical rather than in absolute terms.

Tapered and re-entrant are opposite failure directions, and the chemistry that causes one often fixes the other. A tapered (positive-taper) profile usually means the mask or passivation layer is eroding faster than the chemistry is cutting straight down, letting lateral etching creep in near the top; a re-entrant (negative-taper, undercut) profile usually means the opposite — insufficient sidewall passivation lets the plasma attack laterally near the bottom, often right at the mask-to-film interface. Pushing more sidewall-protecting polymer deposition tends to straighten a taper but risks tipping it toward re-entrant if overdone — the two names sit on either side of the same knob.

Bowing is named separately from taper because its cause is geometric, not chemical. A tapered or re-entrant profile usually traces back to a passivation chemistry imbalance that an engineer can push one way or the other with a gas-ratio knob. A bowed profile, by contrast, typically comes from ions scattering off the sidewall partway down a deep, narrow feature and etching sideways at that depth specifically — a geometric consequence of aspect ratio, not a chemistry setting, which is why it gets a name of its own instead of being folded into "tapered."

The names exist because the consequence downstream is different for each one. A tapered contact can still land correctly but with reduced bottom contact area; a re-entrant profile risks voiding or pinching off the very next film that has to fill it, since deposition into an overhanging mouth is far harder than filling a straight or tapered hole. Reading "re-entrant" on a cross-section report is a direct warning about the next deposition step, not just a comment on the etch step that produced it.

Read a sidewall profile by its name rather than by its angle number alone, and the diagnosis is already half-written: vertical confirms the etch is on target, tapered or re-entrant point to which direction the passivation chemistry needs to move, and bowed points away from chemistry entirely and toward the feature's aspect ratio — the same four words an etch engineer uses to triage a cross-section photo before ever opening a recipe file.