Photolithography

Photolithography is the combination of photography and lithography. Its uses include the mass printing of photographs. Microphototolithography is the use of photolithography to transfer geometric shapes on a photomask to the surface of a semiconductor wafer for making integrated circuits.
Manufacturing.
Photolithography makes integrated circuits such as memory and central processing units.
A special light-and-shadow pattern is shone through a photomask, onto a sheet or wafer of silicon that had been covered with a material called a photoresist. The light hardens the photoresist. When the board is dipped into a special acid, the parts that had no light on them are dissolved away. 
Memory chips have thousands or millions of identical cells. Because is much easier to make than a CPU, (a large die with a non-repetitive structure), RAM chips are far less expensive than processors.