How we manufacture silicon: computers' crucial ingredient not found in nature

Letter #12

A letter to Greta Thunberg
by Katie Singer

Dear Greta,

Could we discuss silicon, that substance on which our digital world depends? [1] Silicon is a semiconductor, and tiny electronic switches called transistors are made from it. Like brain cells, transistors control the flow of information in a computer's integrated circuits. Transistors store memory, amplify sound, transmit and receive data, run apps and much, much more.

One smartphone (call it a luxury, hand-held computer with portals to the Internet) can hold more than four billion transistors on a few tiny silicon chips, each about the size of a fingernail.

Computer chips are made from electronic-grade silicon, which can have no more than one impure atom per billion. But pure silicon is not found in nature. Producing it requires a series of steps that guzzle electricity [2] and generate greenhouse gases (GHGs) and toxic waste.

Silicon's story is not easy to swallow. Still, if we truly aim to decrease our degradation of the Earth and GHG emissions, we cannot ignore it.

Step One

Silicon production starts with collecting and washing quartz rock (not sand), a pure carbon (usually coal, charcoal, petroleum coke, [3] or metallurgical coke) and a slow-burning wood. These three substances are transported to a facility with a submerged-arc furnace.[4]

Note that transporting the raw materials necessary for silicon production between multiple countries, via cargo ships, trucks, trains and airplanes uses oil and generates greenhouse gases. [5]

Step Two

Kept at 3000F (1649C) for years at a time, a submerged-arc furnace or smelter "reduces" the silicon from the quartz. During this white-hot chemical reaction, gases escape upward from the furnace. Metallurgical-grade silicon settles to the bottom, 97-99% purenot nearly pure enough for electronics. [6]

If power to a silicon smelter is interrupted for too long, the smelter's pot could be damaged. [7] Since solar and wind power is intermittent, they cannot power a smelter.

Typically, Step Two takes up to six metric tons of raw materials to make one metric ton (t) of silicon. A typical furnace consumes about 15 megawatt hours of electricity per metric ton (MWh/t) [8] of silicon produced, plus four MWh/t for ventilation and dust collection; and it generates tremendous amounts of CO₂.[9]

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