Eli5: How are they able to make transistors so extremely small?
In: 11
They use a process called “photolithography” to modify a piece silicon by adding carefully controlled impurities and plating on metal wires.
Pure silicon doesn’t conduct electricity, but by adding specific impurities you can alter its electrical properties. There are a variety of different types of transistor design, but one of the older ones is made by having two areas of silicon with “n” impurities, and a third area of silicon with “p” impurities in between. Connect a wire to each area of silicon and you have a transistor.
Photolithography works by using a projector (like a regular video or film projector) to project a design pattern onto a piece of silicon. The difference between a lithography projector and a cinema projector, is that a cinema projector is designed with a magnifying lens – but a lithography projector is designed with a minifying lens. This allows you to take a big design drawing and project it at tiny size onto the silicon.
The trick with photolithography is that before the design is projected on – the silicon is coated with a light activated varnish (called a photoresist). Liquid photoresist is painted on. Then the design is projected. Where the light hits the photoresist , it sets hard. Where the photoresist is in a dark part of the design, it stays liquid. Then the silicon is washed, and the liquid photoresist gets washed off, but the hard photoresist is left forming a barrier. Then impurities are added – the photoresist gets in the way, so only areas of silicon without photoresist are affected. Then there is a chemical wash which removes the hardened photoresist , going back to flat silicon.
The process is then repeated with different design patterns to add different impurities or to spray on metal wires.
Latest Answers