photolithography
Photolithography is how you print a circuit onto a silicon wafer with light. You coat the wafer in a light-sensitive film called photoresist, shine light through a stencil-like mask that carries one layer's pattern, and the exposed resist changes — it either washes away or stays behind, leaving a sharp template. It is like making a photographic print: the mask is the negative, the resist is the paper, and light decides what survives. Etch through the openings, strip the rest, and the pattern now lives in the chip.
A chip is built layer by layer, so you repeat this dance dozens of times — transistors first, then the wiring stacked above them — each layer aligned to the last within a few nanometers. The catch is resolution: light cannot draw features much finer than its own wavelength, so chasing smaller transistors meant ever-shorter light, from ultraviolet down to today's extreme ultraviolet (EUV) at 13.5 nm. EUV is absorbed by almost everything — even air — so it runs in vacuum and steers by mirrors, not lenses, with the mask itself a mirror rather than a transparent stencil. This is the engineering heart of every advanced fab.
A common confusion: the named "node" (5 nm, 3 nm) is now a marketing label, not the light's wavelength or any single physical dimension on the chip. Engineers beat the wavelength limit long ago with tricks like immersion lithography, optical proximity correction, and multipatterning — splitting one fine layer into several coarser exposures. EUV finally lets a single exposure do what once took four, but each ASML EUV scanner costs well over $150 million, and the newest High-NA machines run past $350 million.