💡 Photolithography (EUV / ASML)

This is, quite literally, printing with light. The wafer is coated with a light-sensitive chemical called photoresist, then a mask carrying one layer's pattern is illuminated and its image is projected — shrunk roughly 4x — onto the wafer's surface. Wherever light hits, the resist's chemistry changes, so the pattern can later be developed away like a photograph. Because features are far smaller than visible light's wavelength, the physics of light itself becomes the limiting factor.

That is why the most advanced layers use Extreme Ultraviolet (EUV) light at a wavelength of just 13.5 nanometers — about 14 times shorter than the older light it replaced. EUV is so energetic it is absorbed by air and ordinary glass, so the entire system runs in a vacuum and uses mirrors instead of lenses. The light is made by blasting tiny falling droplets of molten tin with a high-power laser ~50,000 times a second, vaporizing them into a plasma that glows at exactly 13.5nm.

Only one company on Earth, ASML in the Netherlands, can build these EUV machines. Each is the size of a bus, contains ~100,000 parts, costs well over $150 million (next-gen 'High-NA' systems approach $400 million), and represents one of the most complex tools humanity has ever made.

The physics: fighting the wavelength of light

Lithography's central limit is diffraction — you generally cannot cleanly print features much smaller than the wavelength of the light you print with. For decades the industry used 193nm deep-ultraviolet light and clawed below that limit with tricks like immersion (printing through water) and 'multi-patterning' (exposing the same layer several times to interleave finer lines). Each trick added cost and steps. EUV at 13.5nm broke the logjam by shrinking the wavelength itself ~14x, letting the smallest features be printed in fewer passes.

Why EUV is borderline miraculous engineering

EUV light is absorbed by air, glass, and almost everything else, so the entire optical path runs in vacuum and bounces off ultra-flat multilayer mirrors (made by Zeiss) polished so smooth that, scaled to the size of Germany, the tallest bump would be under a millimeter. The light itself is born by firing a high-power CO2 laser at ~50,000 falling tin droplets per second, each blasted twice into a plasma glowing at exactly 13.5nm. Only ASML can integrate this into a working machine, making it arguably the most complex tool ever mass-produced.

History and the hardest challenges

EUV took roughly three decades and tens of billions in shared R&D before reaching volume production around 2019. The persistent challenges are throughput (enough wafers per hour to be economical), 'stochastic' defects (because so few EUV photons land per feature, random shot-noise causes broken or bridged lines), and mask defects that print onto every wafer. These failure modes are a major reason advanced-node yield starts low and matures slowly.

Why this matters for AI chips specifically

Every doubling of transistor density that lets an AI chip hold more compute and on-chip memory ultimately traces back to lithography resolution. Leading-edge process nodes for GPUs and accelerators are simply not manufacturable in volume without EUV — which is why control of EUV is so geopolitically charged, and why TSMC's early, aggressive EUV adoption helped it pull ahead in making the world's AI silicon.