UNIT 9 • METALS AND ATOMIC LAYERS

Sputtering: knock metal off a target

In a vacuum chamber, a plasma of argon ions is driven into a block of metal, the target. Each impact knocks atoms loose, and they fly across and land on the wafer as a thin film. No chemical reaction is needed, so any metal or alloy that can be made into a target can be deposited.

Sputtering is atomic-scale billiards, and it works for almost any metal.

WHAT GETS SPUTTERED

The metals a fab lays down

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Aluminum

Classic chip wiring for decades, and still used for bond pads and top layers

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Titanium and TiN

Thin adhesion and barrier layers under tungsten and aluminum

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Tantalum and TaN

Barrier that keeps copper from wandering into silicon and insulators

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Copper seed

A thin copper skin that copper plating grows from

Many of the thinnest metal layers in a chip are sputtered.

EVAPORATION

Evaporation vs sputtering

Evaporation Heat a source in vacuum until it vaporizes and coats the wafer

Sputtering Knock atoms loose with a plasma. Better adhesion and coverage, and the production workhorse

Lift-off Evaporation's straight-line beam suits lift-off patterning in research labs and MEMS

Shared limit Both are line-of-sight, so both struggle to coat deep, narrow trenches evenly

Good for flat surfaces and lab work, but they cannot reach into the deepest features.

COPPER PLATING

Fill the wires by electroplating

Cu²⁺ + 2e⁻ → Cu

Copper wiring is deposited by dipping the wafer in a copper sulfate bath. A sputtered copper seed layer carries the current, and copper ions plate onto it. Chemical additives make the fill start at the bottom of each trench and climb upward, so the wires fill without voids.

Plating fills the copper wires from the bottom up.

ATOMIC LAYER DEPOSITION

One atomic layer per cycle

ALD alternates two gases with purges in between. Each gas reacts only with the surface until every site is used up, then the reaction stops by itself. Every cycle therefore adds one uniform layer, and you repeat cycles until the film is thick enough.

Self-limiting reactions give ALD its atomic-level control.

ALD TRADEOFFS

Perfect films, slowly

Thickness control About 0.1 nm per cycle, so roughly 20 cycles make a 2 nm film

Conformality Coats deep, narrow trenches and the sides of 3D fin transistors evenly

Speed Slow, so it is reserved for thin films that must be perfect

Used for High-k gate dielectrics like HfO₂, spacers, and thin barrier layers

ALD trades speed for control, and modern transistors are worth the trade.

UNIT 9 STUDY COMPLETE

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Ready for the Fab Challenge?

You can compare sputtering, copper plating, and atomic layer deposition.