Etch Stack Naming Convention

A typical etch film stack is named from the top down by what the plasma sees first, not by what the engineer cares about most — photoresist, then spacer, then the functional layer being patterned (polysilicon, aluminum metal, or an oxide), then whatever sits underneath it as an etch stop. The "typical names" an etch engineer uses on a process flow chart are really a shorthand for role, not chemistry: "spacer" names a function (protect a sidewall), "sacrificial oxide" names a lifespan (gone before the device ships), and "thermal oxide" names a formation method (grown, not deposited). Reading a stack by name tells you what each layer does before you ever look at its thickness.

A few things explain why these particular names stuck across the industry rather than some other label:

Photoresist is named for what it is, not what it becomes. It is the only layer in the stack that is never part of the finished device — it exists purely to be patterned by light and to protect what's underneath during one etch step, then gets stripped. Every other layer's name describes a permanent or semi-permanent role; photoresist's name describes a temporary job.

Spacer describes geometry, not material. A "spacer" can be silicon nitride or silicon dioxide depending on the flow — the name survives material changes because it always refers to the same shape: a thin sidewall sleeve left behind after an anisotropic etch strips a conformal film off horizontal surfaces, leaving it only on vertical edges. That's why you'll hear "nitride spacer" and "oxide spacer" as sub-names under the same umbrella term.

Thermal oxide versus sacrificial oxide is a formation-and-fate distinction, not a thickness one. Both can be silicon dioxide, both can be thin, and both can even be grown the same way. The difference the typical names encode is intent: thermal oxide is grown to stay (gate dielectric, field isolation), while sacrificial oxide is grown or deposited specifically to be stripped — often to clean a surface, round a corner, or protect it through one implant step before a wet etch removes it entirely. The same recipe, two different names, because the names track what happens next, not what happens now.

Etch process flow is the sequence that reads these names in order. A real flow doesn't etch "a stack" in one step — it etches one named layer at a time, switching chemistry at each boundary: fluorocarbon chemistry to cut through oxide, halogen chemistry to cut through polysilicon, chlorine-based chemistry for aluminum metal. The stack's names are effectively the chemistry changelog; an engineer reading a flow chart knows the gas box will switch recipes at every named-layer transition, because each name implies a different etch stop signal and a different selectivity target against the layer below it.

Read a film stack by its typical names rather than by thickness alone, and the process flow explains itself: photoresist and sacrificial oxide are built to disappear, spacer is built to hold a shape regardless of what it's made from, and thermal oxide, polysilicon, and aluminum metal are built to survive into the finished part — the naming convention is the etch engineer's fastest way to answer "what happens to this layer next" without reading a single spec sheet.