In short: The etching process selectively removes material from a wafer to transfer a patterned shape into the layer beneath a mask. Wet etching uses liquid chemistry and is largely isotropic, while dry (plasma) etching uses reactive gases to achieve the anisotropic, vertical profiles that modern transistors demand. Selectivity — how much faster the target etches than the mask and underlayer — is what keeps the pattern faithful.
In semiconductor fabrication, the etching process is the step that physically removes exposed material from a wafer so that the pattern defined in photoresist (by lithography) becomes a permanent feature in an underlying film — silicon, silicon dioxide, polysilicon, metal, or nitride. Lithography draws the map; etching carves it. Wherever the resist mask is open, material is removed; wherever the resist protects the surface, material stays.
Every integrated circuit passes through dozens of etch steps. Getting them right determines gate width, contact depth, trench profile, and ultimately whether billions of transistors switch correctly. Three variables govern the outcome: the etch chemistry (wet or dry), the profile (isotropic or anisotropic), and the selectivity between the film being etched, the mask, and the layer below.
Wet etching immerses the wafer in a liquid chemical that dissolves the exposed film. Classic examples include buffered hydrofluoric acid (BHF) for silicon dioxide, phosphoric acid for silicon nitride, and potassium hydroxide (KOH) or TMAH for silicon. The reaction proceeds in three stages: reactants diffuse to the surface, react to form soluble products, and those products diffuse away.
Because a liquid attacks the surface equally in every direction, wet etching is usually isotropic — it etches sideways under the mask at roughly the same rate it etches downward. That lateral attack, called undercut, widens features and limits wet etching to layers where exact edge placement is not critical. Its strengths are high throughput (many wafers per batch), low equipment cost, and excellent selectivity, since a well-chosen chemistry can attack one material while barely touching another.
One important exception: KOH etching of crystalline silicon is anisotropic with respect to the crystal planes, etching <100> planes far faster than <111>, which produces the sloped, well-defined cavities used in MEMS and some sensors.
Dry etching replaces the liquid with a low-pressure plasma of reactive gases such as fluorine- or chlorine-bearing species (CF4, SF6, Cl2, HBr). An RF field ionises the gas and accelerates ions vertically toward the wafer. This combines two mechanisms: a chemical component (radicals react with the surface to form volatile products that are pumped away) and a physical component (energetic ions sputter and bombard the surface). Reactive Ion Etching (RIE) blends both, and because the ion flux is directional, material is removed mostly straight down — giving the anisotropic, vertical profiles essential for dense, nanometre-scale devices.
Variants tune the balance between chemistry and bombardment. ICP (inductively coupled plasma) decouples plasma density from ion energy for high rate and control. Deep Reactive Ion Etching (DRIE) — the Bosch process — alternates etch and passivation steps to cut extremely deep, high-aspect-ratio trenches for MEMS and through-silicon vias.
Selectivity is the ratio of the etch rate of the target film to the etch rate of another material present — usually the mask or the layer underneath. A selectivity of 20:1 of oxide to silicon means the oxide is removed twenty times faster than the silicon stop layer, so the etch can clear the oxide and halt before damaging what lies beneath.
High selectivity matters most when etching a thin film that sits on a layer you must preserve (for example, etching a gate oxide while sparing the silicon channel), and when the mask is thin and must survive to the end of the etch. Wet chemistries generally offer the highest selectivity; dry etching trades some selectivity for the directional control that fine geometry requires. Process engineers tune gas mixture, pressure, power, and temperature to hit the needed selectivity and profile simultaneously — and that balancing act is the heart of real etch development.
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Explore CourseTron Courses →Wet etching uses liquid chemicals and is usually isotropic, undercutting the mask, while dry (plasma) etching uses reactive gases and directional ions to produce anisotropic, vertical profiles. Wet is cheaper with higher selectivity; dry gives finer resolution and better edge control.
Transistors are packed at nanometre pitch, so features must have straight, vertical sidewalls to keep their intended width. Isotropic undercut would widen and distort them, which is why anisotropic dry etching dominates advanced nodes.
Selectivity is how much faster the target material etches compared with the mask or the underlying layer. High selectivity lets the etch clear one film and stop cleanly on the next, preserving the structure beneath.
Isotropic wet etching is still valuable for removing sacrificial or blanket layers, cleaning, and MEMS cavity formation — cases where exact sidewall geometry is not critical and its high selectivity and throughput are an advantage.
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