UNIT 10 • ETCHING

Remove what the mask does not protect

1 · Mask A patterned layer, usually photoresist, covers the areas to keep

2 · Etch Chemistry attacks the exposed film and removes it

3 · Strip The mask is removed, leaving the pattern in the film below

Coming later How the mask gets its pattern is the job of lithography

An etch copies the mask's pattern into the film beneath it.

PROFILES

Two ways an etch can eat

An isotropic etch attacks in every direction, so it undercuts the mask by about as much as it goes deep. An anisotropic etch attacks mainly straight down and leaves vertical walls. Tiny, closely spaced features need anisotropic etching.

Undercut is the enemy of small features.

WET ETCHING

Chemistry in a bath

HF (hydrofluoric acid) Dissolves SiO₂ quickly and barely touches silicon

Hot phosphoric acid Strips silicon nitride while leaving oxide mostly alone

KOH Etches silicon along its crystal planes. Common in MEMS, and an exception to the isotropic rule

Character Very selective and cheap, but most wet etches are isotropic and hard to control at small sizes

Safety HF is highly toxic and burns deeply, so fabs handle it with special training

Wet etches are selective and simple, and they undercut.

DRY ETCHING

Plasma etching: chemistry plus bombardment

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Plasma

Splits a gas into reactive radicals and ions

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Ion bombardment

An electric field drives ions straight down, so the walls come out vertical

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Chemistry

Radicals react with the film to form volatile products that are pumped away

Reactive ion etching (RIE) combines a chemical reaction with physical ion impact.

RIE is both chemical and physical, which is why it can be both selective and directional.

SELECTIVITY

How well does the etch spare what you want to keep?

selectivity = R_target / R_kept

Here R is an etch rate: the target film versus the layer you want to keep. A 20:1 selectivity means the etch removes the target 20 times faster than the layer beneath it, so that layer loses 1 nm for every 20 nm etched. Gate etches need very high selectivity because the oxide underneath can be only a nanometer or two thick.

High selectivity is what lets an etch stop cleanly.

ETCH GASES AND ENDPOINT

Match the gas to the film, then watch the plasma

Fluorine gases (CF₄, CHF₃) Etch silicon dioxide and silicon nitride

Chlorine or bromine (Cl₂, HBr) Etch silicon, polysilicon, and aluminum

Oxygen plasma Ashes away photoresist after the etch

Endpoint Sensors watch the color of the plasma's light, which changes when the film clears

Choose gas by film, and use the plasma itself to know when to stop.

UNIT 10 STUDY COMPLETE

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Ready for the Fab Challenge?

You can explain how wet and dry etches differ and what selectivity means.