What differentiates the microprocessor from its predecessors constructed out of valves, individual transistors or small integrated circuits is that it brought us, for the first time, a complete processor on a single chip of silicon.

Silicon is the basic material from which chips are made. It is a semiconductor which, when doped with impurities in a particular pattern, becomes a transistor, the basic building block of digital circuitry. The process involves etching the transistors, resistors, interconnecting tracks and so forth onto the surface of the silicon.

First a silicon ingot is grown. This must have a defect-free crystalline structure, an aspect which places limitations on its size. In the early days, ingots were limited to a diameter of 2in, although 8in is now commonplace. In the next stage, the ingot is cut up into slices called wafers. These are polished until they have a flawless, mirror-like surface. It is these wafers on which the chips are created. Typically dozens of microprocessors are made on a single wafer.

The circuitry is built up in layers. Layers are made from a variety of substances. For example, silicon dioxide is an insulator, and polysilicon makes up conducting tracks. When bare silicon is exposed, it can be bombarded with ions to produce transistors – this is called doping.

To create the required features, layers are added to cover the entire surface of the wafer, and the superfluous portions are etched away. To do this, the new layer is covered with photoresist, onto which is projected an image of the features required. After exposure, developing removes those portions of the photoresist which had been exposed to light, leaving a mask through which etching can take place. The remaining photoresist is then removed using a solvent.

This process continues, a layer at a time, until a complete circuit is built up. Needless to say, with the features being made measuring less than a millionth of a metre across, the tiniest speck of dust can create havoc. Particles of dust can be anywhere from one to 100 microns across – three to 300 times the size of a feature. Microprocessors are manufactured in clean rooms – ultra-clean environments where the operators wear space-age protective suits.

In the early days, semiconductor manufacturing was hit and miss, with a success rate of less than 50% of working chips. Today, far higher yields are obtained, but nobody expects 100%. As soon as all the layers have been added to a wafer, each chip is tested and any offenders are marked. The individual chips are now separated, and at this point are called dies. The faulty ones are discarded, while the good ones are packaged in Pin Grid Arrays – the ceramic rectangles with rows of pins on the bottom which most people think of as microprocessors.

The 4004 used a 10-micron process: the smallest feature was 10 millionths of a metre across. By today’s standards, this is huge. For example, a Pentium Pro under these constraints would be about 5.5in x 7.5in, and would be slow; fast transistors have to be small. By 1998 most processors used a 0.25-micron process. Both Intel and AMD had reduced this to 0.13-micron by 2002, with 0.1-micron remaining the mid-term goal.

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