Silicon is the second most common element in the Earth’s crust, after oxygen. Almost all of it is locked up with oxygen, in quartz and in the other silicate minerals that make up most rocks. Quartz, pure silicon dioxide, is the tough one: it survives long after other minerals have worn away, which is why it makes up so much of the sand on most beaches. So it seems odd to call any sand valuable. And yet the silicon inside a computer chip is among the purest materials humans have ever made, and the journey from beach to chip is long, hot and surprisingly fragile.
Silicon and oxygen hold on to each other very tightly. To separate them, quartz is heated with carbon, usually in the form of coal, coke and wood chips, in an electric arc furnace at temperatures of around 2,000 °C. The carbon takes the oxygen away as carbon monoxide, and molten silicon collects at the bottom.
What comes out is called metallurgical-grade silicon, and it’s about 98 to 99 per cent pure. That sounds pure. For most uses it’s perfectly good: much of it goes into aluminium alloys, which become engine parts, and into silicones, the rubbery materials used in sealants and kitchenware. But for electronics, one per cent of impurities is a disaster. A chip relies on adding tiny, precisely controlled amounts of other elements to silicon, a process called doping, and stray atoms of iron or aluminium ruin that control.
The next stage turns the silicon into a gas so it can be cleaned. The most common method, called the Siemens process, reacts the silicon with hydrogen chloride to make trichlorosilane, a liquid that boils at a low temperature. Liquids can be purified by distillation, the same principle used to separate alcohol from water, and trichlorosilane is distilled again and again until almost nothing else is left.
The purified gas is then passed over hot silicon rods, where it breaks down and deposits fresh silicon, layer by layer. The result is polysilicon, and for electronics it reaches purities of 99.9999999 per cent or better, often described as “nine nines” or higher. At that level, there is fewer than one foreign atom for every billion silicon atoms.
Why so fussy? Because silicon is a semiconductor, and semiconductors are extraordinarily sensitive to stray atoms. In a perfect silicon crystal, every atom shares its four outer electrons with four neighbours, and those electrons are locked into bonds, so almost none are free to carry a current. Pure silicon is a poor conductor. But slip in a single atom of phosphorus, which has five outer electrons, and one is left over with nowhere to go, free to wander. A few such atoms in every million is enough to make silicon conduct many thousands of times better. Chipmakers rely on exactly this, adding carefully chosen impurities in precise amounts to build every transistor. But that only works if they start from silicon in which the accidental impurities are far, far rarer than the ones they add on purpose. Nine nines is not perfectionism. It’s a blank page.
Polysilicon is made of many small crystals jumbled together, and chips need a single, unbroken crystal. To make one, the polysilicon is melted in a crucible and a small seed crystal is dipped into it, then slowly rotated and pulled upward. The silicon freezes onto the seed, copying its arrangement of atoms exactly. This is the Czochralski method, named after the Polish chemist who discovered it in 1916, reportedly after dipping his pen into molten tin instead of his inkwell.
The finished crystal, called an ingot or boule, can be two metres long and weigh over 100 kilograms. It is sliced into wafers less than a millimetre thick, polished until they are flatter than almost any other manufactured surface, and only then sent to a chip factory, where the real work of building circuits begins.
There’s a twist in all this. The crucible that holds the molten silicon has to be made of quartz too, and it must be extremely pure, or it will contaminate the crystal it is meant to protect. Very few places in the world have natural quartz pure enough to make these crucibles easily. The best-known is near the small town of Spruce Pine, in North Carolina in the United States, which has supplied much of the world’s high-purity quartz for decades.
When Hurricane Helene flooded the area in September 2024, the mines were shut for a few weeks, and the semiconductor industry suddenly paid attention to a place most people in it had never heard of. Supplies held up, partly because companies kept stocks and other sources exist, but it was a reminder of how much of modern technology rests on a few unusual rocks.
Then the real precision begins. Inside a chip factory, each wafer goes through hundreds of steps: coated with light-sensitive chemicals, exposed to patterns of ultraviolet light, etched, cleaned, and layered with other materials, over and over, until billions of transistors sit on each chip. A single 300-millimetre wafer can carry hundreds of chips, depending on their size, and all of it depends on the crystal underneath being almost perfectly pure.
Stepping back, it’s easy to think of computers as being made of code and clever design. They are. But they are also made of sand, coal, electricity, very hot furnaces and a long chain of chemistry, all aimed at one goal: taking one of the most common elements on the planet and making a piece of it more perfectly ordered than almost anything else on Earth.