Photolithography

Photolithography is patterning by light: a wafer is coated with photoresist, a chemical that changes solubility where light hits it, and an image of the desired circuit is projected onto it through a mask and lenses. Develop the resist and the pattern becomes a stencil for etching, doping, or depositing material — one layer of a chip; repeat dozens of times, aligned to fractions of a wavelength, and a three-dimensional circuit emerges. Jay Lathrop and James Nall coined the term in the late 1950s while shrinking circuits for the U.S. Army, borrowing photographic emulsion techniques and running a microscope's optics in reverse to print small.

Every earlier way of building circuits made one component at a time; photolithography makes all of them at once. Because light has no moving parts, shrinking the pattern makes chips simultaneously faster, cheaper, and less power-hungry — the engine behind Moore's Law and six decades of exponentially cheaper computation. Features have shrunk from tens of micrometers to a few nanometers: modern chips are printed at the scale of viruses, with billions of transistors defined in each exposure sequence.

Lathrop and Nall coated a germanium wafer with Kodak photoresist, exposed it through a mask using a metallurgical microscope turned upside down so it demagnified instead of magnified, and etched where the resist had been removed. Fairchild and Texas Instruments industrialized the process within three years, and it became the heart of the planar process for integrated circuits. Each generation since has been an optics arms race — brighter sources, shorter wavelengths, absurdly perfect lenses and mirrors — culminating in extreme-ultraviolet machines whose mirrors are polished to atom-scale smoothness.

Photolithography is how integrated circuits, microprocessors, memory chips, and image sensors are physically made — the manufacturing step beneath the entire digital world. Its economics (copies are nearly free once the mask exists) created the semiconductor industry's structure, from foundries to fabless design. Computers, phones, satellites, and machine learning all trace their existence to printing with light.