The transformation spreads from the wires' point of contact. Credit: source:

A new technique for producing silicon might make this technologically vital element cheaper. It could also give engineers new ways to design silicon chips.

The process, developed by Toshiyuki Nohira and colleagues at Kyoto University in Japan, uses electricity to strip oxygen from silica, the natural oxide of silicon1. The method could make large amounts of silicon from mineral silica, such as the quartz that makes up the bulk of sand.

It could also turn a small patch of silica to silicon, a technique that could ultimately be important in semiconductor technology. A computer chip is a network of silicon structures, which conduct electricity, embedded in layers of silica, which don't. These structures are carved into wafers of pure silicon, parts of which are converted to silica with oxygen.

But it is not possible to convert silica back to silicon on a chip. A two-way switch could make chip design and manufacture more flexible.

Extreme measures

Making silicon industrially requires extreme and expensive conditions. Silica is melted in a furnace at around 1,700 °C and is reacted with carbon to produce impure silicon. The high-quality silicon in chips is made by melting chemically purified silicon and letting it recrystallize slowly.

Nohira and his colleagues can convert silica to silicon without melting. They immerse a piece of silica - a quartz plate, say - in a bath of molten calcium chloride salt at 850 °C, and pass an electric current through a metal wire touching the quartz.

Where metal touches silica, the oxygen atoms in the silica become oxide ions, which dissolve in the molten salt - and the silica turns slowly into silicon. The newly created silicon conducts electricity into the block, and the transformation spreads, like a growing mould, from the point where the wire makes contact.

To convert part of a silica slab, one only has to touch the end of the wire to that point. If the metal tip is fine enough, the process could be limited to a microscopic area. The researchers have yet to demonstrate the process at this scale, however.

Silicon made in this way is pitted and brittle, so it would have to be reprocessed into high-quality crystalline silicon. But with current processes this has to be done anyway. And the researchers say that they can potentially reduce the processing temperature to 500 °C by using a molten mixture of salts. This would make the process less likely to damage silicon chips.

The method might also be used to manufacture other important pure elements - such as zirconium - from their recalcitrant natural compounds, says Nohira's team.