Polysilicon

Polysilicon is silicon so pure that, in the best grades, about one atom in a billion or fewer is something else. The 'poly' means it's made of many small crystals packed together, rather than one big one. It comes as rough gray rods or chunks.

Most of it is made by the Siemens process, developed in the 1950s. Inside a bell-shaped steel reactor stand pairs of thin silicon rods, joined at the top like an upside-down U. An electric current heats them to about 1,100 °C. A gas mixture of purified trichlorosilane and hydrogen flows past. At the hot surface, the trichlorosilane breaks down: the silicon stays on the rod, while the hydrogen helps carry the chlorine away as hydrogen chloride. Over several days, the rods grow from pencil-thin to many centimeters thick. They're then broken into chunks, cleaned in acid and packed in clean rooms.

Chip-grade polysilicon is the starting point for the silicon ingot: it's melted and regrown as a single perfect crystal. Most polysilicon made today goes into solar cells, which use a slightly less pure grade.