October 2005




Silicon chlorides are some of the cornerstone chemicals of the Information Age. Trichlorosilane (HSiCl3) is an intermediate compound used to produce extremely pure silicon, from which computer chips, the "brains" of electronic devices and high-tech toys, are manufactured. The famous high-tech industrial area of California, Silicon Valley, was named for the chemical element that has revolutionized the way we communicate and play. And chlorine has played an essential role in the high-tech revolution.

Silicon: Abundant and Valuable

trichlor_vaseSilicon is the third most abundant element in planet Earth, after iron and oxygen. It occurs combined with other elements, especially oxygen, in most of the rocks, sands and soils at the Earth's surface. Silicon is present, for example, in the chemical make-up of minerals such as feldspars, quartz, micas and clays. For thousands of years people have used these natural resources to make pottery and bricks (from clays), glass and cement (from sands) and countless other necessities and conveniences of life.


In the modern age of electronics, scientists have identified silicon as an ideal semiconductor of electricity. What does that mean? Electricity can be thought of as a flow of electrons-the negatively charged particles in atoms. All matter can be divided up into three categories: electrical conductors, in which electrons are free to flow (e.g., most metals), electrical insulators, in which electrons are too tightly bound to flow (e.g., most non-metals) and semi-conductors, such as silicon, in which electrons are relatively tightly bound until some change in their environment causes them to flow freely. Semiconductors can be made to control the flow of electrons through electronic devices.


Sand: A Natural Starting Point

Quartz sand, a breakdown product of the weathering of the continents, is the starting point for manufacturing pure silicon chips. To be a semiconductor, silicon must be made to be unnaturally pure, a process carried out in super-clean settings, sometimes known as "clean labs."

The Steps: Sand Silicon

1. Separate silicon from oxygen

This is accomplished by heating the sand grains of SiO2 with carbon, C, at the very high temperature of about 2,000 ºC. The reaction that occurs is:

Trichlorosilane 1

The purity of the Si at this stage is at about 90%.

2. Get silicon into trichlorosilane

Hydrochloric acid, HCl, is chemically combined with Si at 300 ºC to produce trichlorosilane and hydrogen gas:

Trichlorosilane 2

3. Heat trichlorosilane until it decomposes into very pure silicon (and other products)

Trichlorosilane is heated to 1150 ºC, the temperature at which it decomposes. One of the products of heating is very pure silicon.

Trichlorosilane 3

The purity of silicon at this stage is about 1 part per billion. That means that there might be one atom of impurity for every billion silicon atoms. To help you visualize one part per billion, imagine just one 4-inch hamburger in a chain of hamburgers circling the earth at the equator 2.5 times!


High-Tech Toys
The silicon computer chip has revolutionized life, even affecting how kids play. Many of today's toys contain computer chips.
How do you know whether a toy contains a silicon chip? According to the Intel website, "A basic rule of thumb is, if a device uses electricity and you can 'tell it what to do' by programming it or customizing it, there's a chip inside."

Follow-up Activities:


1. The American Chemical Society, a 158,000 member organization of chemists, celebrates National Chemistry Week (NCW) October 16 - 22. This year's theme, "The Joy of Toys," invites the public to explore the role of science and chemistry in the creation of toys. The Society's president, Dr. Bill Carroll, is planning an NCW Extreme Tour of 15 cities in 10 days. Follow his adventures by reading his blog as he takes chemistry on the road, starting October 14.

2. List one of your favorite toys from the past (or present) and investigate the chemistry that must have been used to manufacture the toy. What natural raw materials were the starting points for these toys? What chemistry was used to transform them?

3. Pure silicon forms the main parts of computer chips, but much more chemistry goes into chip construction. For more information on silicon chip construction, see: http://nobelprize.org/physics/educational/integrated_circuit/history/


For a list of previous "Chlorine Compound of the Month" features, click here.


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