Before the fab the wafer
From sand to wafer.
Every chip starts as a slice of one perfect crystal: a single lattice of silicon atoms, unbroken for two metres, grown slowly out of a pot of liquid metal.
This page follows silicon from quartz to the mirror-flat disc that enters a fab. Scroll to go on. Drag a scene to turn it.
Melt temperature from SUMCO, crystal length and wafer thickness from Wikipedia. Scenes are schematic and not to scale unless they say so.
01 How clean is clean
Counting the nines.
Purity is counted in nines. Solar silicon is 7N to 10N; silicon for chips is 10N to 11N. At 11N, one atom in a hundred billion is something other than silicon.
This cube holds 24,000 atoms. Slide the purity up and watch the red ones, the foreign atoms, disappear long before the chip industry is satisfied.
Grades as reported by Bernreuter Research. The counts are arithmetic: 24,000 × 10−N.
02 Quartz to polysilicon
Growing rods out of gas.
Quartz is silicon dioxide. Heated with carbon, it gives up its oxygen and leaves metallurgical-grade silicon, about 98% pure: fine for aluminium alloys, useless for chips.
So it is turned into a gas, trichlorosilane, distilled, and turned back into silicon. In the Siemens process, thin silicon filaments inside a bell jar are heated to about 1,150 °C. The gas, mixed with hydrogen, deposits pure silicon on them, layer on layer, until pencil-thin wires become rods about 120 mm thick.
Filaments of 7 mm grow to about 120 mm in roughly two days, per Wikipedia. The rods are then broken into chunks: polysilicon, pure but made of many small crystals pointing every which way. Bernreuter notes that fluidized-bed reactors, the main alternative, use about a tenth of the electricity.
03 The Czochralski method
Pulling a crystal from a pot.
Polysilicon chunks go into a quartz crucible and melt at about 1,420 °C, under argon. Boron or phosphorus is added in exact amounts to set how the silicon will conduct.
Then a small seed crystal touches the surface and is drawn slowly upward, turning. Atoms freeze onto it in the seed's own lattice, so the whole ingot becomes one crystal.
Drawn to scale for a 300 mm crystal with a 2 m body; time is compressed. A crystal that size weighs about 330 kg, by arithmetic from silicon's density. The quartz wall slowly dissolves, so Czochralski silicon carries about 1018 oxygen atoms per cm³. Where that matters, float-zone growth skips the crucible.
04 Ingot to wafers
A saw made of wire.
The ingot is ground to an exact diameter, and a small notch is cut along its side to mark the crystal's orientation. Every wafer will carry that notch; every tool in the fab uses it to line the wafer up.
Then a wire saw cuts it: one long wire, wound many times around grooved rollers into a web, running at speed through an abrasive slurry or carrying diamond grit. The whole ingot is pressed through the web and comes out as a stack of wafers in one pass.
Schematic: this web has 60 wires, a real one far more. Every cut turns a slice of silicon into dust, which is why wire keeps getting thinner.
05 Grind, etch, polish
Flatter than anything you own.
A sawn wafer is rough, and the saw leaves damage just under its surface. It is ground or lapped flat, etched in acid to remove the damaged layer, then polished with a slurry on a turning pad until it is a mirror.
The result: a disc 300 mm across and 775 µm thick, weighing about 125 g. It has to be that flat: a scanner's focus is shallower than a micrometre, so a hill or valley on the wafer blurs the print.
Dimensions and weight from Wikipedia. After cleaning and inspection, wafers are packed 25 to a box and shipped to the fab.
06 Where to go next
Ready for a thousand steps.
From here the wafer enters a fab, where it will spend months being printed, etched, filled and polished again, layer upon layer, until it holds hundreds of chips.
- Anatomy of a fab: the building the wafer goes into
- Writing with light: how each layer is printed
- What a chip costs: how many chips one wafer is worth
Every figure comes from a publisher named below, or is arithmetic from their figures and says so. The machines shown are generic, not any company's design.