The Integrated Chip

The concept behind an integrated chip is relatively simple: an entire electrical circuit with numerous transistors, wires, and other electrical devices all built into a single square of silicon.  These chips are smaller than a centimeter-by-centimeter square, yet they can hold  millions of transistors. If one person sat down to build all those miniscule parts and then connect them, it would take a whole year. But companies turn out several million integrated chips every few seconds -- that's about the time it took you to read this sentence. 

The reason integrated chips are possible at all is because engineers learned ways to build layers, making mllions of transistors across the chip all at the same time.  The first ideas on how to build the chips were developed by Jack Kilby and Robert Noyce in 1958, and they've been developed further over the years. 

The chip is built upwards, layer by layer. Each layer is made by putting masks with particular patterns over the silicon and then altering the qualities of the silicon -- or perhaps putting down metal or insulators -- in the exposed parts.  It's as if you could build a house by laying down a pattern which covered the entire foundation except for where the outside walls were supposed to go.  Sprinkle some bricks all over and suddenly there are walls.  Next you'd lay down another pattern which has holes only where the inside walls and the furniture are supposed to be.  Sprinkle wood all over the house and now there are wooden walls and tables and chairs.  Other patterns might allow you to lay down porcelain for the bathrooms, pipes for the heat, and, as a final step, shingles for the roof. 

Of course, the chip isn't built of wood and bricks and porcelain, it's made out of a semiconductor crystal. The chip starts out as a thin wafer of P-type silicon.  This is then coated with a layer of silicon dioxide -- kind of a silicon rust, which doesn't conduct electricity. On top of this is placed a chemical called photoresist.  Flashing a pattern of light (like the grid of light and dark that's formed by a window screen) on the photoresist turns any parts exposed to the light hard.  The bits left in shadow stay soft. When an etching chemical is applied those soft parts, and the silicon dioxide underneath them, are removed. The hard photoresist is then dissolved, leaving a pattern of raised silicon dioxide along the surface.  Since the silicon dioxide doesn't conduct electricity, it keeps different parts of the final circuit separated from others. 

Following the same method, a pattern of polysilicon (which does conduct electricity and is part of the transistor) is added.  Then, again using projected photoresist masks, areas of the chip are doped to become N-type silicon, another crucial part of a transistor.  Lastly, metal leads are added to connect the various components on the chip. 

Since the chips are so small, hundreds are made on a single silicon wafer at once.  After all the patterns have been faithfully reproduced on to the chips, the wafer is sliced up into individual chips. 

Resources: 
-- The Way Things Work by David Macaulay 
-- Van Nostrand's Scientific Encyclopedia 
-- http://webopedia.internet.com/TERM/i/integrated_circuit_IC.html

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