EUV Lithography
The extreme-ultraviolet light process used to print the smallest, most advanced features onto a chip — made by exactly one company in the world, ASML.
Lithography is how a chip's circuit pattern gets printed onto silicon: light is shone through a mask to etch a pattern, the same basic idea as a stencil, at a scale of nanometers. Below a certain size, ordinary light's wavelength is simply too long to draw features that small accurately, so the industry moved to EUV — light with a wavelength of about 13.5 nanometers, hard enough to generate and focus that it took decades and tens of billions of dollars to make it work at all.
That difficulty is exactly why only one company makes EUV machines at all, why every leading-edge foundry in the world — TSMC, Samsung, Intel — depends on that single supplier, and why export restrictions on EUV equipment have become one of the sharpest tools in the U.S.-China contest over advanced chipmaking.