The electronic brains that run the world

The silicon chip in your mobile phone is not perfect because it is a single crystal like diamond, even though it is; nor because it is ultra pure, even though it is. No, it is perfect because it is an orchestrated collection of defects; it is these that give your phone its immense social power.

Silicon chips are the electronic brains that run the world. They fly our planes, drive our trains, take care of the washing while we are out, keep us alive in hospital and handle our most intimate conversations. We get all this from a thin sliver of material the size of a postage stamp that was invented in the 20th century when the popularity of the telephone and the radio led to a general enthusiasm for all things electronic.

Harnessing electricity to do more complicated things like computation relied then on thermionic valves that were essentially souped-up electric light bulbs. They were hot, unreliable and bulky. But they could act as switches, turning electricity on and off, providing the ones and zeros of the digital world and so when connected formed the first programmable computers such as Colossus, which was pivotal for the British in the second world war. After the war, it became clear that connecting yet more values and other electronic components to get more powerful computers was not going to work. The engineers encountered something they called the “tyranny of numbers”, which meant that as the machines got more complex they became more unreliable.

They needed a way to make computers simpler but more powerful. The answer was to make all the electronic components of a computer out of one material: silicon.

Silicon, a semiconductorIt is a semiconductor, which means that it conducts electricity, but not very well. This sounds like a problem, but is exactly what you want if you are trying to create a computer out of a single material because you need some parts to conduct electricity and others not to conduct at all. Engineering the material to do this involves adding tiny amounts of impurities. Silicon has four outer electrons, which are bound up in the chemical bonds holding the crystal together. By adding a tiny amount of phosphorous, which has five outer electrons, you in effect add a free electron to the crystal and make it conduct moderately well. Similarly you can add boron, which has only three outer electrons and in effect do the same thing, only now the conducting charge is called an electron hole.

If you put a phosphorus silicon layer next to a boron silicon layer, the holes and the electrons cancel each other out at the junction but create an electric field that means that electrons like to flow in only one direction across the junction. This is called a diode, but the computational magic happens by adding another phosphorus layer, which creates a diode that will allow electrons to flow only if the voltage applied to the sandwich layer is just right. This is an electronic switch, called a transistor. It does what thermonic valves do but is smaller than a hair on a flea.

Being able to make electronic components like resistors, diodes and transistors out of a single piece of silicon and connecting them all yielded the archetype of the integrated silicon chip that we have today. But hindsight is a wonderful thing and none of the pioneers believed it would work, because initially it didn’t solve the tyranny of numbers. 

The chip still involved thousands of components linked together, albeit out of a single material, but if there was one part that was in error the whole chip didn’t work — some ways it was worse because these parts being so tiny could not be repaired. The solution to the problem turned out to be cleanliness. Chip technology relies on tiny amounts of defects being introduced into the silicon, such as phosphorus or boron atoms, but any other impurities are severely detrimental. So the starting point for all silicon chips is extremely pure silicon, what is called 9 nines silicon, which is 99.9999999% pure. 

Silicon chips are made in a clean room environment where workers have to wear special suits and must enter and exit via an airlock. The craft of these silicon makers is not so much about learning technical skills, but their ability to be precise, calm and methodical in an alien environment more akin to a spacecraft than a traditional workshop.

A powerhouseBut even purest cleanest silicon isn’t free enough of defects. This is because solid silicon forms crystals, billions of them, and where these crystal meet are yet more defects. The solution is to rid silicon chips of the crystal boundaries completed by making every chip out of a giant single crystal. The Czochralski process is used to do this: pure silicon is melted in a crucible, then a tiny seed crystal is lowered into the crucible to touch the surface of the liquid and removed slowly. As it moves away from the surface of the molten silicon, liquid solidifies on to the seed, growing an immense single crystal of silicon often more than two metres in length. You may have visited some wonderful crystal grottos in your life, but they are nothing compared to the wonders that assault your eyes in a silicon fabrication facility.

The crystals are sliced into thin ultra pure 9 nines silicon wafers ready to be manufactured into silicon chips by the selective addition of impurities, etching with acid, and using a photographic method to create the interconnections. It is the industrial scale of the operation that makes silicon chips so cheap, and the precision of the silicon makers that makes them so reliable. Over the years the power of each individual chip has increased as the size of the transistors has become smaller. It is this ability to miniaturise the fabrication of precise amounts of impurities that has made silicon such a powerhouse: there is more computing power in a modern smartphone than the Apollo spacecraft that landed on the Moon. If any material proves that defects are necessary for perfection, it is undoubtedly silicon.