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Silicon is a key component of virtually all modern electronics.

Stuff of Progress, Pt. 4: Silicon

By Tony Morley @tonymmorley

Silicon is a lustrous, semi-conductive metallic element used to make high performance steel, aluminum, photovoltaic solar cells and, of course, computer processor chips. After oxygen, silicon is the most abundant element contained within the Earth’s crust,  accounting for nearly 30 percent of the crust by mass. However, metallic elemental silicon is not found in pure form within nature. Silicon is typically bound with oxygen in mineral form. This kind of silicon is found in abundance as silicon dioxide (SiO2), quartzite, quartz, as regular beach sand, and as hundreds of other mineral compounds. Swedish chemist Jöns Jacob Berzelius was, in 1824, the first to isolate and describe elemental silicon. However, the ability to isolate and manufacture high-purity metallic silicon in bulk would not be achieved until much later.

Humans have been transforming the silicon dioxide found in sand into glass for nearly five thousand years, steadily improving our ability to do so. The production of silicon-based glass transformed our collective ability to create art, glassware, and later develop the optics that would help to found the ideas of the Scientific Revolution, circa 1540 to 1650. The use of glass in early scientific instruments revolutionized scientific understanding, aiding in major discoveries in physics, astronomy, chemistry and biology. That continued with the advent of improved glass blowing and the greater range of more advanced instruments it enabled. Gaining momentum during the Industrial Revolution, improved glass technology deployed in microscopes, telescopes, thermometers, and an innumerable plethora of laboratory instruments, transformed our understanding of the natural world.Glass continues to provide an invaluable service to the advancement of industrialized civilization, from the high-strength and scratch-resistant glass found in billions of mobile phones and computer displays; to the protective glass used for the windows of our homes, offices and vehicles; and the more commonplace glass used in a million other applications.

Humanity produces roughly 6.7 million tonnes of silicon annually. Approximately 80 percent is produced as ferrosilicon for use in the manufacture of iron and high performance steels with high-temperature and corrosion-resistant properties, or used in alloying with aluminum. Alloying of aluminum with silicon improves its strength and corrosion resistance, while improving the casting characteristics of the metal, and is commonly used in engine casting and production.

A very small minority of the high purity silicon dioxide mined annually—approximately 15 percent—is destined to become the feedstock for the silicon wafers that make up the central processing units of civilization’s billions of mobile phones and computer systems.

The first working silicon transistor was developed in 1954 at Bell Labs by an American physical chemist Morris Tanenbaum. The transistor was rather coolly received at the time, and few could have accurately speculated how dramatically silicon transistors would transform the progress and growth of civilization. Processor- grade polycrystalline silicon (a highly purified form of metallic silicon) is the base material of the silicon wafer boards upon which central processing units are built. It is, however, a common misconception that the silicon in computer chips is made from the sand of any random beach. While it is true that silicon chips are made from sand, it is by no means ordinary sand.Silicon dioxide in the form of sand fills our beaches, blankets the floor of river deltas and oceans, and covers our deserts. All the sand in Saudi Arabia will, however, not furnish civilization with the silicon for even a single microprocessor. In order to reach the astonishingly high levels of silicon purity required, the source material must itself be of unparalleled purity.

Such sources of ultra-pure silicon dioxide are mined in just a few locations globally, perhaps most notably a group of small mines in Spruce Pine, Tennessee. The white, open cut mines of high purity white silicon dioxide are so radiantly bright as to shine like a beacon when viewed from Google Earth. Once mined, the silicon dioxide source rock must be washed, crushed, filtered to remove impurities, packaged and prepared for smelting. The pure elemental silicon must be liberated from the oxygen bonds through ultra high temperature smelting in a powerful electric arc furnace. The silicon dioxide is mixed with two key sources of carbon, typically coal and wood, and put into the furnace, where the added carbon helps carry off the oxygen in the form of carbon dioxide gas. What is produced is 99 percent pure elemental silicon metal.

Unfortunately, from the perspective of the silicon computer processor, 99 percent purity looks like a filthy wasteland. What is required is a silicon ingot with a purity of 99.99999999999 percent. That is one non-silicon atom for every seven billion silicon atoms. That’s equivalent to one person out of the entire human population.

In order to reach this astonishing level of purity, the metallic silicon is subjected to a series of chemical processes. These processes convert the silicon metal into silicon tetrachloride – a compound needed to produce high performance fiber optic glass that transports 99 percent of the world’s intercontinental communications data across some 285 undersea data cables. The secondary product, trichlorosilane, is subsequently converted into polysilicon, the ultra-pure silicon that will be transformed into silicon computer wafers. From there, the polysilicon is melted under high temperature and in an inert gas atmosphere, within a pure quartz crucible. The molten crucible is maintained in rotation and a high-purity silicon seed crystal is inserted, onto which the molten silicon in the crucible begins to grow. As the crystal grows, it is carefully drawn – upward and out – from the molten mass in an extremely slow continuous motion. Once complete, a single crystal of pure silicon has been grown, with a mass of approximately 100 kg, or 220 lbs.

The ingots are precision-sliced and polished to a flawless mirror finish. The most perfect of these circular disks of pure silicon are the medium onto which is etched the computer processor architecture of modern CPUs. Other wafers are converted into the base material of solar cells.

The quality, purity and physical properties of finished silicon wafers haven’t changed dramatically over the last decade. However, the microprocessor architecture etched upon that silicon has improved a great deal. Silicon is a perfect example of a material resource with little intrinsic value, until such time as humans build into the material the capacity to do useful work. Over the coming decades, silicon will continue to provide the key to our digital age. Silicon serves as a testament to civilization’s capacity to transform a seemingly invaluable mineral resource into one of the most technologically advanced and valuable products humanity created.

Tony Morley is a freelance science writer and progress and growth exponent, based in Sydney, Australia.


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