Ready For the Metal Round
Although chips are made of many layers, the transistors are all near the bottom. The upper layers are for the metal connecting wires. That means the size of a chip is determined mostly by the number of transistors it has, not by the amount of wiring it requires. That's what pushes semiconductor manufacturers to constantly shrink the size of their transistors: The smaller the transistor, the smaller the chip (or the more transistors you can fit on a same-sized chip). The smaller the chip, the more you can fit on a wafer. The more chips on a wafer, the more you can sell for the same amount of work. Moore's Law is just good old capitalism at work.
After the transistors are built up on our chip, it will go through the same process several more times to build up the “metal layers.” These are the copper or aluminum wires that connect the transistors together. The amount of wiring depends on the complexity of the chip. (Remember the houses-and-utilities puzzle from Chapter 3, “How Chips Are Designed”?) Some chips might have only three or four layers of wiring; complex chips might have eight or more.
The process of applying metal is the same as for the other layers that were put on earlier. First, the entire wafer is covered with an insulating material that runs into all the cracks and grooves of the chip (it won't be smooth anymore). Then a thin layer of metal, either aluminum or copper, is applied on top of that. Finally, a layer of photoresist covers the metal.
Tech Talk
Around 1996, the whole process of using copper wiring in chips got a lot of coverage in the press. It's not clear why this happened, because this abstruse facet of chip manufacturing was of little interest to the general public and of even less interest to those in the industry, who had known about it for some time. Copper certainly makes a better electrical conductor than does aluminum, as any electrician or homeowner will confirm.
The major reason copper wasn't adopted sooner was cost. Although chips use a tiny amount of copper, the billions of chips produced each year add up to a significant drain on this limited resource. Gold is an even better conductor than copper, and perhaps future “designer chips” will flaunt their gold wiring.
Now the wafer goes under another stepper with a different piece of film. Again the stepper exposes the photoresist, and again it sets up and protects the metal we just applied. After it's washed away, the net result is that some of the metal remains, although most of it will be eaten away, leaving very thin metal lines or wires. Depending on which company is making the chip, there might be gaps underneath the metal wires. Other companies use a slightly different process that leaves material under the wires for support.
This process will be repeated several more times, each time building up another layer of metal wires atop the previous layer. Each layer of this elaborate cake is separated by a layer of insulating material so that wires crossing over each other don't touch and short out.