The Evolution of Microchips

Since 1960, the number of elements that could be manufactured in an IC has doubled every 18 months. State-of-the-art silicon chips in 1979 contained 29,000 transistors, but by 1996 this figure had risen to 5.5 million. As every new generation of silicon chip squeezes more components into a circuit, electronic devices become smaller. When portable computers first became commercially obtainable, they were the size of a suitcase. Now, modern lap-top computers are the size of A4 notepads, and they have far higher processing and memory capabilities.

Some silicon chips have just one function. Memory chips in computers, for example, are created solely to store and retrieve information. Other silicon chips have many functions and act like minicomputers in their own right. They are known as microprocessors or microchips and contain extremely complex integrated circuits.

Microchips can be taught, or programmed, to do a variety of activities, including controlling the actions of other pieces of electrical equipment. In many modern cars, for instance, microchips monitor the engine's temperature and pressure, and adjust the amount of fuel injected into the engine accordingly. Typical silicon chips are between five and seven millimetres square and they're thin enough to pass via the eye of a needle. Every chip can include thousands of miniature circuits connected by tiny tracks of conductive aluminium, copper or tungsten.

Inside the chip

Pure silicon has a crystal structure similar to diamond, and is electrically insulating. Nevertheless, if impurity atoms (dopants), for example phosphorus, are implanted into its crystal structure, silicon becomes a semiconductor - it conducts tiny electrical currents. When silicon is doped, it is given either a negative or a positive charge, and is known as n- or p-type silicon, depending on its charge. Transistors, capacitors and resistors are made by combining patterned sections of this doped silicon with layers of conductive and insulating materials.

Every layer of material is built up in particular places on the chip by masking off sections of wafer that don't need to be covered - in much the same way as a decorator uses masking tape to protect glass from stray splashes of paint. Integrated circuits include tens of thousands of different elements linked together by circuit tracks. Often, it takes at least 50 procedure steps to make a microprocessor, and their manufacture is known as Large Scale Integration (LSI).

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