Wednesday, 16 May 2018

Computer chips out of sand

Sand is very rich in the element silicon. This substance can have a very pure grid form, which gives interesting electrical properties. Such a grid consists of almost only silicon atoms. Some of these atoms can be replaced by boron or phosphorus, resulting in semiconductors or transistors in the grid.

Due to a special melting and coagulation process, the silicon atoms grow into an almost perfect grid. Solidified silicon is drawn out of a melt bath into the form of a cylinder and is cut into thin discs. These discs are called ‘wafers’ and are sold to chip manufacturers for further processing. They can make multiple chips from such a wafer, which are cut from the wafer in the end.

At present billions of transistors fit on one chip. On the basis thereof, electronic circuitry and circuit combinations are designed on the computer. These integrated circuits are printed on a 'mask', which is a glass plate with the full design of the wafer in chrome. Such a mask resembles a slide: if light shines through it you can see the image. The image on the mask must be transferred to a wafer. This is done in several steps: photolithography, etching and deposition. These techniques are too complicated to describe here in just a few words.

The above steps are not done once, but sometimes several to dozens of times. In this way, slowly but surely, all sorts of complicated 3D structures are made in and on the wafer. Metal layers are added, holes again etched, other layers added, etc. Always again with a new mask.

As a final step follows the cutting of the chips out of the wafer, after which a ceramic or plastic housing is made around it. Small wires run from the chip to pins that can connect the chip to the outside world. All these operations are carried out in a so-called cleanroom. This is necessary because the smallest dust particle will destroy the end result.

A waver with electronic circuitry

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