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UNIT 11 • PLANARIZATION (CMP)

Layers pile up bumps

Without CMP layers copy the bumps focus plane After CMP flat, easy to focus on focus plane

Every patterned layer leaves topography behind. Left alone, bumps stack up, thin wires crack when they cross steps, and lithography cannot keep a bumpy surface in focus, because its depth of focus is only a fraction of a micron and shrinking.

A flat wafer is a lithography requirement, not a cosmetic one.

CHEMICAL MECHANICAL POLISHING

Grind flat with chemistry and grit

down-force carrier rotating platen wafer, face down polishing pad platen slurry everything spins high spots wear away first

The wafer is pressed face down onto a spinning polyurethane pad while a slurry of tiny abrasive particles in a chemical solution flows between them. The chemistry softens the surface, the abrasive removes it, and high points wear away faster than low ones.

CMP levels a surface by wearing down the high spots first.

CMP INGREDIENTS

Four things set the polish

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Pad

Porous polyurethane that carries slurry across the wafer

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Slurry

Abrasive particles (silica, ceria, alumina) in a tuned chemical solution

⚖️

Pressure and speed

Down-force and rotation set how fast material comes off

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Endpoint

Motor current or optical sensors show when the film has cleared

Recipe, pad, and slurry decide how flat and how fast.

CMP IN THE FAB

Where the polish happens

Shallow trench isolation Polishes the oxide overfill flat and stops on a silicon nitride layer
Copper damascene Removes the excess copper, leaving metal only in the trenches
Tungsten plugs Polishes tungsten off the surface, leaving plugs in the contact holes
Interlayer dielectric Flattens the insulator before the next metal layer goes on
CMP is what makes many stacked wiring layers possible.

DISHING AND EROSION

How polishing goes slightly wrong

ideal flat surface Dishing wide copper line dips Erosion dense array wears down

Soft copper in a wide line polishes lower than the surrounding insulator, which is called dishing. Dense arrays of lines wear the insulator away faster than open areas, which is called erosion. Designers add dummy fill patterns so the pattern density stays even across the die.

Even pattern density gives even polishing.

CMP DEFECTS

Polishing leaves a mess

Scratches Hard particles or pad debris gouge the surface
Residual slurry Abrasive left behind becomes a particle defect
Corrosion Slurry chemistry can attack exposed copper
Post-CMP clean Scrubbing and rinsing right after polishing removes slurry before it dries
Every polish is followed by a clean.

UNIT 11 STUDY COMPLETE

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Ready for the Fab Challenge?

You can explain why wafers are polished flat and how CMP can go wrong.