Photolithography is a modern development of an older process used in printing. In lithography, smooth pieces of limestone had oil-based images burned by acid into their surface. The non-oily portions were then sealed by gum arabic. Oil-based ink then only adhered to the unsealed areas so that complicated pictures and typed regions could be reproduced.
Photolithography is used in the mass production of transistors and electronic components. In April 1957 Jay Lathrop and James Nail of the U.S. Army's Diamond Ordnance Fuse Laboratories in Maryland produced the first electronic components that did not require manual soldering. The required design is often pre- formed on a photo-mask, consisting of a series of opaque chromium lines on glass. The aim of the process is to transfer the pattern of the mask onto the flat surface of a wafer of silicon. The silicon is cleaned and covered with a photo-resistant substance. Ultraviolet light is then shone through the mask, and this light chemically changes the photo-resistant substance. A developing chemical then washes away the chemically altered material leaving the underlying material exposed. The whole process is similar to that of producing a photographic negative. Projection printing photolithography is only capable of producing a small final image, but images can be stepped over the surface of the wafer. The final step bakes the photoresistant material to the surface. Then a new layer of material is added and the process is repeated. The whole series of electrical components: and the connecting wiring is built up layer by layer.
Photolithography has advanced to the smallest scale in microlithography and nanolithography, where it is used in the production of today's highly complex microchip cores for computers and cell phones.
