Lesson 13.1Micro & nanoChapter 13 · Micro- and nanofabrication

Czochralski crystal growth (silicon wafers)

Pulls a giant single crystal of silicon slowly out of a melt

Czochralski growth (say "cho-HRAHL-skee") makes a giant single crystal of silicon: one solid piece in which every atom sits in the same perfect grid. A small seed is dipped into molten silicon and pulled up very slowly, and the silicon freezes onto it. Sliced into thin disks called wafers, these crystals are the base of almost every computer chip and of many solar cells.

Photo: ArticCynda, CC0, via Wikimedia Commons

Czochralski crystal puller in cross-section · 6 stepsCircled numbers match the steps below. The top panel is a cut through the furnace (the puller); the bottom panel shows the finished crystal.

How it works

Melt the silicon. Chunks of ultra-pure silicon are melted in a quartz crucible (a heat-proof bowl) at about 1,420 °C. A measured pinch of boron or phosphorus (a dopant, which sets how the silicon conducts electricity) goes in too, and argon gas keeps air away from the melt.
Dip the seed. A small seed crystal, cut so its atomic grid points in a chosen direction, is lowered until it just touches the melt. Silicon atoms freeze onto it and copy its grid.
Pull a thin neck. The seed is first pulled up quickly, making a neck only a few millimeters wide. Defects (dislocations, flaws in the atomic grid) grow out through the neck instead of spreading into the crystal.
Grow the shoulder and body. The pull slows, so the crystal widens into a cone (the shoulder), then grows at full diameter, typically 200 or 300 mm. Seed and crucible turn in opposite directions to keep the melt even, and the crystal rises at roughly a millimeter per minute.
Taper the tail and cool. At the end, the diameter is narrowed into a cone so the crystal leaves the melt without a thermal shock (a sudden temperature change that would damage its grid). The finished crystal, called an ingot, is lifted out and cooled slowly.
Grind and slice. The ingot is ground to an exact cylinder and notched to mark its crystal direction. Diamond-coated wire slices it into wafers under 1 mm thick, which are then polished to a mirror finish.

Key facts

Materials: Mostly silicon; also germanium and some oxide crystals for lasers and optics
Typical parts: Cylindrical ingots (also called boules), sliced into wafers
Part size: Typically 200–300 mm in diameter, up to about 2 m long and several hundred kg
Precision: One continuous atomic grid without dislocations; diameter held to about a millimeter before grinding; finished wafers flat to about a micrometer (a thousandth of a millimeter)
Surface finish: As grown: shiny metallic gray with faint ridges along its length; finished wafers are mirror-polished
Batch size it suits: Mass production: each puller grows one ingot at a time, around the clock
Tooling and machine cost: A puller costs on the order of a million dollars; the quartz crucible is a consumable, replaced after one or a few ingots
Speed: Roughly 1 mm of crystal per minute, so one ingot takes a day or more

Strengths and limits

Strengths

Huge, nearly perfect single crystals
Exact doping sets the electrical behavior
Mature and highly automated, up to 300 mm wafers
The standard starting point for chips and many solar cells

Limits

Oxygen from the quartz crucible dissolves into the silicon
Slow: one ingot can take days
Uses a lot of energy and consumes crucibles
The dopant level drifts from one end of the ingot to the other

Where it's used

ElectronicsProcessors and memory chips, made on 300 mm wafers
Solar energyMonocrystalline solar cells
Power electronicsPower transistors and diodes in chargers, cars and trains
Sensors and MEMS (tiny machines etched in silicon)Phone camera image sensors, motion sensors (accelerometers)
Optics and lasersGermanium for infrared optics, laser crystals such as Nd:YAG

How to recognize it

Mostly, you can't. The crystal ends up invisible inside every chip: a small square cut from a wafer, sealed in a black package on the circuit board of your phone or laptop.
Solar panels. Monocrystalline cells look evenly dark, without a flaky pattern, and often have slightly cut-off corners: they were squared off from a round ingot.
Bare wafers. A mirror-like gray disk, usually with a small notch or flat on its edge that marks the crystal direction.
Whole ingots, in factory photos or museums, look like giant gray bullets with faint straight ridges along them that reveal the crystal's internal grid.

Compare with

Single-crystal casting1.8also grows one crystal from a seed, but freezes a shaped metal part inside a mold instead of pulling a free cylinder.Molecular beam epitaxy13.3adds new crystal layers on top of a finished wafer, a few atoms at a time, instead of growing the bulk crystal.Photolithography13.2the next step for most wafers, printing circuit patterns onto them.

Watch it

Linton Crystal Technologies · Seeding, necking and pulling, animated inside a puller

Intel · Where the ingot fits on the path from sand to finished chip

Linton Crystal Technologies · Real crystal pullers and finished ingots in a factory

Linton Crystal Technologies · How an ingot is sliced into wafers

See real photos

Wikipedia: Czochralski method — process illustrations and photos of silicon ingots
Wikimedia Commons photo search

Remember

A seed pulled slowly from molten silicon grows into one flawless crystal up to about 2 m long.
A thin neck, slow pulling, opposite rotations and argon gas keep that crystal perfect and clean.
Almost every chip and many solar cells start as a slice of such an ingot.