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why → how

Why EUV lithography blasts tin droplets with lasers

Every leading-edge chip is patterned by a machine that, tens of thousands of times per second, vaporizes a falling droplet of molten tin with a high-power laser. The setup is absurd — and there is no other way to make 13.5nm light.

Science intro May 4, 2026

Why it exists

The phone in your pocket has a CPU built on a process node somewhere between 5 and 3 nanometers. The features on that chip — the wires, the transistor gates — are smaller than a single virus particle. To pattern them, the chip industry needs to project light through a mask onto a silicon wafer the way an old slide projector throws an image on a wall. The catch is the wavelength. To resolve features that small, the light has to be smaller than the features. DUV at 193nm — the workhorse for two decades — runs out of steam around 7nm features even with multi-patterning tricks. The next step down on the spectrum that anyone could plausibly engineer was extreme ultraviolet at 13.5nm: roughly fourteen times shorter than DUV.

Then the problems start. There is no laser that emits coherent 13.5nm light. There is no transparent material at 13.5nm — every glass, every plastic, every gas, even air, absorbs it. Lenses are out. Even the mirrors don’t work as ordinary mirrors; aluminium reflects almost nothing in this band. And whatever source you build has to throw enough photons per second to expose hundreds of wafers an hour, because chips have to be cheap.

The solution that ASML and its supplier Cymer landed on, after roughly two decades of development, is the kind of thing you’d write off as a joke if a sci-fi novel proposed it. Drop a tiny droplet of molten tin into a vacuum chamber. Hit it with a low-power laser pulse to flatten it into a pancake. Hit the pancake with a megawatt-class CO₂ laser pulse. The tin vaporizes into a plasma so hot it glows in the EUV band — including, importantly, a useful peak right around 13.5nm. Catch that light with a stack of multilayer mirrors, bounce it through the system, and project the image. Repeat the whole sequence roughly fifty thousand times per second.

That’s how every cutting-edge chip on Earth is currently patterned.

Why it matters now

Without EUV, the industry would be stuck. The roadmaps that take logic from 7nm down to 5, 3, and 2nm — the nodes that current AI accelerators, modern phone SoCs, and the latest CPUs ride on — all assume EUV. The performance and density story behind everything from GPU training clusters to the M-series Mac in front of you is downstream of EUV being a working production technology, not just a research project.

It also matters geopolitically. ASML, a Dutch company, is the only manufacturer of EUV scanners in the world. Each machine is a refrigerator-sized vacuum system that costs in the low hundreds of millions of dollars and ships in pieces on multiple cargo aircraft. Export restrictions on these machines are now a primary tool in the U.S.–China semiconductor dispute precisely because there’s no second source: if you can’t buy an ASML EUV scanner, you can’t make leading-edge logic, full stop.

The short answer

EUV = 13.5nm light + tin-droplet plasma source + mirror-only optics in vacuum

The tin droplet exists because no laser produces 13.5nm light directly — but a hot, dense plasma does, and tin happens to have a strong emission line right at the wavelength the multilayer mirrors are tuned for. The mirrors-only optics exist because no material is transparent at 13.5nm. The vacuum exists because air absorbs it. Each strange ingredient is forced by the wavelength, and the wavelength is forced by the feature size.

How it works

The full system has roughly four parts: source, optics, mask, and wafer stage. The source is the part that earns its reputation.

The source: laser-produced plasma

A reservoir of molten tin sits at the top of the source vessel and drips droplets — each on the order of tens of micrometers across — into a vacuum chamber. The droplets fall in a precisely timed stream toward a focal point.

When a droplet reaches the right position, two things happen in fast succession:

  1. A pre-pulse from a smaller laser hits the droplet and flattens it into a thin disk roughly perpendicular to the main beam. This dramatically increases the surface area the main pulse will see — without this step, the main pulse couldn’t deposit enough energy fast enough to make a useful plasma.
  2. The main pulse — a multi-kilowatt CO₂ laser firing in nanosecond pulses — hits the disk and ionizes it into a plasma at temperatures comparable to the surface of the sun. The plasma’s emission spectrum has a strong peak in the EUV band; among the photons it throws off, a useful number land near 13.5nm.

This sequence repeats about 50,000 times per second. (I’ve seen this figure cited consistently in ASML and Cymer materials; the exact rep rate depends on the system generation.) The 13.5nm photons are collected by a “collector mirror” at the back of the source chamber, focused into a beam, and passed into the rest of the optical system.

The byproducts are a problem. Each pulse spits tin debris into the chamber — droplets that didn’t fully vaporize, plasma ions, neutral tin atoms — that would coat the collector mirror and ruin its reflectivity within hours. Mitigating this is its own field: hydrogen gas flow to sweep ions, magnetic and electrostatic fields to deflect them, sacrificial layers, and constant cleaning routines.

The optics: mirrors all the way down

Once you’ve made 13.5nm light, you can’t use lenses to focus it because every transparent material absorbs at that wavelength. You can’t even use ordinary mirrors, because at 13.5nm, the reflectivity of bulk metals is near zero.

The trick is a multilayer Bragg reflector: alternating layers of molybdenum and silicon, each only a few nanometers thick, deposited with sub-angstrom precision. Light reflects weakly from each boundary; if you size the layers right, those weak reflections add constructively for 13.5nm photons hitting at the right angle. A well-made Mo/Si multilayer mirror reflects roughly 70% — which sounds fine until you remember that an EUV scanner uses around 10 mirrors in series, and 0.7^10 is about 3%. Most of the 13.5nm light you generated never reaches the wafer. Brute power at the source compensates.

The whole light path runs in vacuum. Even nitrogen absorbs EUV strongly enough to matter; air is opaque.

The mask and the wafer

The mask itself is a reflective Mo/Si multilayer with the chip pattern etched in an absorbing material on top. EUV light bounces off the unpatterned regions and is absorbed by the patterned regions, projecting an image of the mask onto the wafer through a stack of demagnifying mirrors (typically 4× reduction). The wafer is coated with photoresist that responds to EUV photons. After exposure, normal lithography steps follow: develop the resist, etch, deposit, repeat.

Why this is so hard to replicate

If you wanted to start a competing EUV scanner company today, you’d need to independently solve: a stable >200 W EUV plasma source with manageable debris, multilayer mirror coatings to ASML’s tolerances, a vacuum optical column that maintains nanometer-level alignment under thermal load, a wafer stage that positions to picometers while accelerating at multiple g’s, a mask handling system that keeps reflective masks defect-free in vacuum, and the systems integration to make all of it work as a production tool with high enough throughput and uptime that fabs will actually buy it. ASML had two decades of head start, billions of dollars of R&D, and Cymer (the source supplier they eventually acquired) doing nothing else. There’s a reason there isn’t a second source.

What I’m not sure about

I’m citing the “50,000 droplets per second” figure from public ASML and Cymer materials — it’s the right order of magnitude and the number you’ll see quoted, but specific systems have different rates and ASML doesn’t publish detailed specs for current generations. Same caveat for source power numbers (often cited around 250 W for production systems) and mirror reflectivity (~70% per mirror is the textbook figure).

I’m also glossing over High-NA EUV, the next-generation system that increases the numerical aperture for finer resolution. The first commercial High-NA tools shipped to customer fabs in 2024–2025; the basic source mechanism is the same, the optics are bigger and even more demanding.

Going deeper