UNIT 4 • GROWING THE CRYSTAL

One lattice, no boundaries

grain boundaries ✗

Polysilicon is a jumble of tiny crystal grains. At every boundary between grains, atoms are misaligned, and those defects scatter charges and ruin transistors. Chips need one continuous crystal, with every atom sitting where the lattice says it should.

A transistor built on a grain boundary is a broken transistor.

THE CZOCHRALSKI METHOD

Pulling a crystal from a melt

Polysilicon is melted in a quartz crucible just above 1,414 °C in an argon atmosphere. A small seed crystal is dipped in and pulled upward slowly while rotating. Silicon freezes onto the seed atom by atom, copying its crystal orientation.

The seed sets the crystal orientation; pull rate and temperature set the diameter.

THE PULL, STEP BY STEP

From melt to ingot

Melt Polysilicon and dopant melt in a quartz crucible

Dip The seed touches the melt surface

Neck A thin neck of a few millimeters is pulled first, to shed dislocations

Crown The crystal widens out to full diameter

Body The constant-diameter body grows at roughly a millimeter per minute

Tail The diameter tapers so the ingot can cool safely

Every step is controlled to keep the crystal flawless.

THE PAYOFF

An ingot the size of a person

⌀ 300 mm × ~2 m

A finished 300 mm ingot can be around two meters long and weigh hundreds of kilograms, and one pull takes days. Its entire volume is one continuous crystal.

One seed crystal's atoms, copied hundreds of kilograms deep.

DOPING THE INGOT

Setting n-type or p-type

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Add to the melt

Boron makes p-type; phosphorus or arsenic make n-type. They're mixed into the melt before pulling

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Segregation

Dopants prefer the melt over the crystal, so their concentration drifts along the ingot

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Resistivity spec

Wafers are sorted by resistivity, in ohm-centimeters, to match what customers ordered

Starting doping sets the background the transistors are built into.

OXYGEN

A crucible that dissolves

quartz → oxygen

The quartz crucible slowly dissolves into the melt, so Czochralski silicon carries oxygen, typically on the order of 10¹⁸ atoms per cm³. Some of it forms tiny clusters that trap metal impurities away from the transistors, but too much causes defects.

Oxygen is both a helpful and a harmful passenger.

CZ VERSUS FLOAT ZONE

Two ways to grow a crystal

Czochralski (CZ) Melt in a crucible · large diameters · some oxygen · the workhorse for chips

Float zone (FZ) A moving molten zone, no crucible · very low oxygen · smaller diameters · power devices and detectors

Which is common CZ makes the large majority of wafers for logic and memory

Pick the method by the diameter and purity the device needs.

UNIT 4 STUDY COMPLETE

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Ready for the Fab Challenge?

You can describe how a crystal is pulled from a melt and how it gets doped.