Computers III: Producing the silicon chip Part I

The driving force behind the computer revolution is the silicon chip. Increasingly tiny chips can be found in most of today's electronic appliances and machines, including calculators, washing machines, and coffee makers. The main constituent of these omnipresent pieces is of course silicon, which is an extremely abundant substance on earth. It is the main ingredient of sand, and this well-known fact may give many people the impression that silicon chips might well come from sand. In fact, only sand with a very high quartz content would be suitable for refining to the level of silicon purity required for the chip. Quartz rock quarries are the source of the silicon used in today's computer chip factories, but the use of sand is certainly foreseeable in the future. This would signify an almost inexhaustible supply of raw materials, but there would be many environmental issues to face before the world's deserts and beaches are indiscriminately plundered to fuel the computer industry.

Silicon is an unusual substance because it can act like a metal and a non-metal. Found in the middle of the periodic table of the elements, sandwiched between the metals to the left and the non-metals to the right, it is one of a very few chemical elements that are called semi-metals. Even among semi-metals, silicon is especially good at behaving in either way. It is called a semi-conductor because its outer electrons can be made to be mobile or fixed to specific atoms. When electrons can move fairly easily through the lattice of atoms in the crystal, as in a metal, and these electrons move charge, and the crystal behaves as an electrical conductor. Usually pure silicon dioxide (sand, glass, or quartz) is arranged such that each atom is bound to adjacent atoms by "sharing" outer electrons, and because of this the electrons are not free to move beyond two atoms. This kind of structure is an electrical insulator like non-metals, which means that it will not conduct electricity.

Tiny impurities can be added to silicon to determine whether it behaves as a conductor or an insulator. A silicon chip contains countless miniscule microprocessors built in layers on a silicon wafer. When pure silicon is exposed to very high temperatures and air, it oxidises to form a fine silicon dioxide layer.

The wafer is coated with a special plastic called photoresist, and this is covered with a fine stencil, called a mask, before being exposed to ultraviolet light. All the photoresist exposed to the light becomes soluble while the rest hardens. An acidic solvent removes the exposed photoresist, and the hardened unexposed part is carefully etched into the desired outline. The remaining ridges of silicon dioxide on the wafer form the circuit patterns to be used in each layer. Subsequent layers are etched on each wafer such that windows exist between layers and form connection paths.

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