Lesson 1's per-layer loop named etch as the step that transfers the resist pattern into the film beneath it. This lesson looks at etch itself: the two broad families, wet and dry, the directionality each one gives, and the idea of selectivity, how an etch tells one material from another.
Reading time: about 8 minutes.
7.1 Isotropic versus anisotropic
An etch is isotropic if it removes material at the same rate in every direction, and anisotropic if it removes material much faster in one direction (normal to the wafer surface, straight down) than sideways. The distinction matters enormously once a feature's height becomes comparable to its width: an isotropic etch deep enough to clear a narrow trench to its target depth will have etched sideways by nearly the same distance, undercutting the resist mask and widening the feature well beyond its drawn width. An anisotropic etch can clear the same depth with comparatively little sideways etch (lateral etch), preserving the feature's drawn width and producing a nearly vertical sidewall.
Wet chemical etching (immersing or spraying the wafer with a liquid etchant) is fundamentally a chemical reaction happening wherever the etchant contacts the film, and chemical reactions have no inherent direction; wet etch is therefore almost always isotropic. This makes wet etch unsuitable for most leading-edge pattern transfer, where feature width is far smaller than film thickness, but it is still used where isotropy is acceptable or even desirable: removing a blanket film uniformly, undercutting a sacrificial layer on purpose, or cleaning residues, which is why lesson 1's per-layer loop includes a dedicated clean step separate from the main pattern-transfer etch.
| Aspect | Wet etch | Dry etch (RIE) |
|---|---|---|
| Mechanism | Liquid chemical reaction | Plasma: reactive radicals plus ion bombardment |
| Directionality | Isotropic, undercuts the mask | Anisotropic, near-vertical sidewall |
| Fine pattern transfer | Unsuitable at leading-edge dimensions | Standard choice |
| Still used for | Blanket removal, intentional undercut, cleaning | Trench/hole/via pattern transfer |
7.2 Plasma basics
Dry etch achieves anisotropy by using a plasma, a partially ionized gas, rather than a liquid. Inside an etch chamber, a process gas is struck into a plasma by an RF field; the plasma contains neutral reactive radicals (which etch chemically, much like a wet etch, and contribute the isotropic component of a dry etch) and positive ions. A separate RF bias applied to the wafer's own electrode accelerates those ions nearly straight down onto the wafer surface. The combination, reactive ion etching (RIE), couples a chemical reaction (which does most of the material removal) with physical ion bombardment (which directs that removal preferentially downward, and can also help by continuously clearing reaction byproducts or a protective sidewall film from the bottom of the trench, while leaving it on the sidewalls). This ion-assisted directionality is the core reason RIE can be far more anisotropic than a purely chemical wet etch.
Etch chemistry is chosen per material: fluorine-based chemistries (for example a fluorocarbon plasma) are common for etching silicon dioxide and silicon nitride; chlorine-based chemistries are common for etching polysilicon, metals and some III-V materials. The right chemistry, gas flows, pressure, RF power and bias together define an etch recipe, tuned per layer for the required anisotropy, rate and selectivity.
7.3 Selectivity
Selectivity is the ratio of the etch rate of the material meant to be removed to the etch rate of a material meant to be left behind, usually the resist mask or an underlying stop layer. A high-selectivity etch removes the target film quickly while barely touching the mask or the stop layer beneath it; a low-selectivity etch risks eroding the mask away before the target film clears, or punching through a stop layer that was supposed to protect what is underneath it.
Selectivity to the mask sets how thick a resist (or hard mask) layer needs to be for a given target etch depth; selectivity to the underlying stop layer sets how much margin the process has against over-etch, intentionally running the etch slightly past the point the target film nominally clears, to guarantee every die on every wafer actually clears even with normal film-thickness and etch-rate variation. A contact etch, for example, is commonly run with significant over-etch time specifically because contact depth across a real wafer is not perfectly uniform, and a contact that does not fully clear to the silicon or metal beneath it is an open circuit.
Selectivity is not a single fixed property of a chemistry alone; it depends on the specific pair of materials involved and on the recipe's own conditions, pressure, RF power, gas ratios, which is exactly why etch recipe development spends meaningful effort characterizing selectivity for the specific film stack a given layer actually uses, rather than assuming a chemistry known to be selective in one film stack will behave the same way against a different mask or stop-layer material.
7.4 Worked example: choosing an etch for a contact hole
Assumptions: a MEOL contact hole must clear a thick inter-layer dielectric down to a thin etch-stop layer sitting directly on the transistor's source/drain region, with a high aspect ratio (hole depth much greater than hole diameter).
- High aspect ratio rules out an isotropic wet etch outright: any meaningful sideways etch at this depth-to-width ratio would merge adjacent contacts or badly oversize the hole.
- A fluorine-based RIE recipe, tuned for high selectivity to both the resist/hard mask above and the etch-stop layer below, is the natural choice: anisotropic for the aspect ratio, selective to the mask so it survives the full etch, selective to the stop layer so an intentional over-etch does not punch through into the transistor underneath.
- Lesson 8 returns to exactly this kind of high-aspect-ratio contact etch as a worked example of aspect-ratio-dependent etching, where the etch rate itself slows as the hole gets deeper.
7.5 Interview questions
What is the difference between isotropic and anisotropic etching? Isotropic etching removes material at the same rate in every direction; anisotropic etching removes material much faster normal to the wafer surface than sideways, preserving the drawn feature width.
Why is wet etch almost always isotropic? It is a chemical reaction happening wherever liquid etchant contacts the film, with no inherent directional mechanism.
How does reactive ion etching achieve anisotropy? A plasma supplies reactive neutral radicals for chemical etching and positive ions that an RF bias on the wafer accelerates nearly straight down, directing material removal preferentially downward.
Define selectivity and explain what it is used for. The ratio of the target film's etch rate to the etch rate of a material meant to be preserved (mask or stop layer); it sets how thick a mask must be for a given etch depth, and how much over-etch margin the process has before punching through a stop layer.
Why is a contact etch typically run with intentional over-etch? Because contact depth varies somewhat across a real wafer due to normal film-thickness and etch-rate variation, over-etch time guarantees every contact actually clears to the layer beneath it, relying on selectivity to the stop layer to avoid damaging what is underneath.
Key takeaways
- Isotropic etch removes material equally in all directions and undercuts the mask; anisotropic etch is directional and preserves drawn feature width.
- Wet etch is almost always isotropic; dry (plasma) etch achieves anisotropy through ion-assisted reactive ion etching.
- Selectivity is the ratio of target-film to preserved-material etch rate; it governs mask thickness requirements and over-etch margin.
- Etch chemistry is matched to the target material: fluorine-based for oxides and nitrides, chlorine-based for polysilicon and many metals.
- High-aspect-ratio features, like contact holes, need anisotropic, highly selective etches, usually with intentional over-etch.