What is extreme ultraviolet (EUV) light, and why is that machine so hard to build?

The news says things like "the most advanced chips need EUV" and "a fab brings in EUV". EUV stands for extreme ultraviolet: light with a wavelength of only about 13.5 nm, which the most advanced scanners use to print the finest patterns on a chip. The technique is called EUV lithography. It is hard because almost everything absorbs this light, and the light itself is hard to make, so the whole machine had to be rethought from scratch.

What does a scanner do?

Exposure uses light to shrink the circuit pattern on a photomask and print it onto the light-sensitive photoresist on the wafer. The full set of steps (coat, expose, develop, then hand off to etch) is in "What is photolithography?". This article covers just one thing: the light.

Why does a shorter wavelength print finer lines?

Light is a wave. Its wavelength is the distance from one crest to the next. The light our eyes can see has wavelengths of roughly 400 to 700 nm.

When light passes through a very narrow gap in the photomask, it spreads out a little. This is called diffraction. The narrower the gap, the more it spreads; spread too much, and the image on the wafer blurs, so two lines can't be told apart. A shorter wavelength spreads less, so fine lines come out sharp.

Think of drawing with a pen: the wavelength is the size of the tip, and a finer tip draws finer lines. How steep an angle of light the lens can take in (its numerical aperture) and many clever tricks help too, but for decades exposure light has kept moving to shorter and shorter wavelengths.

How different is EUV from the light used before?

Light	Wavelength
Light we can see	about 400–700 nm
Violet-blue light (g-line)	436 nm
Ultraviolet (i-line)	365 nm
Deep ultraviolet	248 nm, 193 nm
Extreme ultraviolet (EUV)	about 13.5 nm

Going from 193 nm to 13.5 nm makes the wavelength more than ten times shorter in one jump (193 ÷ 13.5 ≈ 14). Extreme ultraviolet is far shorter than ordinary ultraviolet and sits close to X-rays.

Before EUV was ready, engineers stretched 193 nm light for a long time. They put a layer of water between the lens and the wafer so the lens could take in steeper light, and they split the finest layers into two or three separate exposures. It worked, but it meant more steps and more chances for the layers to be misaligned. With EUV, some of those layers can be printed in one go.

What makes it hard? One: even air absorbs it

Extreme ultraviolet is absorbed very easily. Even air swallows it before it gets far. So the whole path the light travels, from the source to the photomask to the wafer, has to be pumped down to a vacuum, and every part along that path has to work in a vacuum.

What makes it hard? Two: glass absorbs it too, so it takes mirrors

Earlier scanners used a set of glass lenses to shrink the pattern and project it onto the wafer, like a camera lens. Extreme ultraviolet is absorbed as soon as it enters glass, so lenses are out. Everything has to be done with mirrors.

But an ordinary mirror hardly reflects extreme ultraviolet either. The answer is to stack two materials, molybdenum and silicon, in alternating layers: dozens of pairs, each layer only a few nanometers thick. Each boundary reflects only a little light, but the spacing between layers is set so precisely that the reflected waves line up crest to crest and add together. It's like a crowd clapping: when everyone claps on the same beat, the sound gets loud. Laying down films a few nanometers thick is everyday work on chips too (see "What is thin film deposition?").

Even so, the best mirror reflects only about 70% of the light; most of the rest is absorbed and becomes heat. On its way from the source to the wafer, the light bounces off several mirrors. If each one keeps 70%, only a little over 10% is left after six mirrors, and only about 3% after ten. The mirror surfaces also have to be extremely smooth, with bumps kept to a few tenths of a nanometer, about the size of an atom.

The photomask changes too. The photomask in the photolithography article is glass that lets light through; an EUV photomask is itself a mirror with the same kind of layer stack, topped with a light-absorbing material shaped into the pattern. The light hits it at a slant and reflects off the uncovered areas, carrying the pattern on to the wafer.

What makes it hard? Three: where does the light come from? A laser hitting tin droplets

Ordinary lamps and lasers can't produce 13.5 nm light that is strong enough. Instead, tiny tin droplets, thinner than a hair, fly one after another through a vacuum, tens of thousands every second. Each droplet is first hit by a weaker laser pulse that flattens it, like a small pancake. Right after, a very strong laser pulse hits it, and the tin heats up to hundreds of thousands of degrees or more, far hotter than the surface of the sun. The atoms lose their electrons, and the tin becomes a plasma: a gas so hot that electrons have left their atoms. This plasma gives off extreme ultraviolet, and a large mirror next to it collects the light and sends it into the machine.

It's a bit like hitting tens of thousands of tiny water drops flying past your eyes every second, with every shot on target. Even with all that effort, only a few percent of the laser's energy becomes usable light. Tin debris also splashes onto the collecting mirror and has to be cleaned off all the time.

Why does the light have to be so strong?

The mirrors eat light all along the way, so not much reaches the wafer. If the light is weak, each field has to be exposed longer, and fewer wafers get printed per hour.

There's one more reason. Light comes in tiny packets called photons. Each extreme ultraviolet photon carries about 14 times the energy of a 193 nm photon, so the same amount of light means more than ten times fewer photons. With so few photons, chance plays a part: some spots happen to get a few more, others a few fewer, and the edges of fine lines come out ragged; in bad cases a line breaks or two lines merge. To make chance matter less, you need more light, which means an even stronger source.

All of this has to work at the same time. That's why EUV lithography was being researched as early as the 1980s but only went into high-volume production in the late 2010s.

With EUV, is the older light no longer used?

It's still used. A chip has dozens of layers, and not every layer is that fine. Only the finest layers use EUV; the other layers keep using deep ultraviolet. For what the "nanometers" in the news mean, see what "2 nm" really means. You can also read What is Moore's law?

Next time you see it in the news

• "A fab brings in EUV": the finest layers in that fab will now be printed with extreme ultraviolet scanners. The other layers still use the older light.
• "The most advanced processes depend on EUV": to print lines that fine with 193 nm light, a layer has to be exposed several times; with 13.5 nm light, some layers can be printed in one go, with fewer steps and fewer chances for mistakes.
• "High-NA EUV": the light is still 13.5 nm, but the mirrors are redesigned to take in steeper light, so lines can be printed a bit finer. The cost is a shallower range in which the image stays in focus.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to the process engineer and tap "Litho" under the name: you are the litho process engineer. You spend every shift around the scanner. In the middle of the night the etch tool downstream goes down, and you decide which lots wait before exposure. In the morning one lot's overlay is out of spec. In the evening, one spot on a photomask prints in the same place in every field. In the same area, the litho equipment engineer (marked Beta) has a night shift where the scanner can't find its alignment marks.

The scanner in the game never says which light it uses, and the litho story never talks about nanometers. The "Know a bit more" part of the Scanner glossary card has two sentences on extreme ultraviolet, and they say the same thing as this article. The companies, people, tool numbers and figures in the game are fictional; why extreme ultraviolet is hard to use comes from public, general sources, and where no single agreed number could be found, only the rough size is given. For the jargon, see Chip fab jargon in plain English.