What is photolithography? How do chip patterns get printed?

Photolithography is the step that uses light to "print" a chip's circuit pattern onto a wafer. In the industry it's called litho for short (in Taiwanese fabs, literally "the yellow light"). It works a lot like developing photos in a darkroom: coat the wafer with a film that reacts to light, shine light through a "negative" with the circuit drawn on it, then wash away the parts that were lit (or the parts that weren't), and the pattern is left behind. Every line and every switch on a chip gets its place this way, one layer at a time.

Why print it at all?

A phone chip holds more than ten billion switches, and its finest lines are thousands of times thinner than a hair. Nobody can draw that one line at a time. The whole pattern has to be made at once, and it has to come out identical on every wafer.

Light can do that. It's like taking a photo: press the shutter and the whole scene lands on the film at once. The word "lithography" originally meant printing from stone ("lithos" is Greek for stone). Put "photo", light, in front of it and you get "printing with light".

What three things do you need?

• Photoresist: a film that reacts to light. Where light hits it, it changes, like the light-sensitive layer on photographic film.
• Photomask: the "negative" for the exposure. A very flat plate of glass (usually quartz) with a light-blocking film on it, shaped like the circuit: where there's film, light is blocked; where there isn't, light goes through. Each layer of the chip has its own photomask.
• Exposure tool (scanner): the tool that shines light through the mask onto the wafer.

Photoresist reacts to blue and ultraviolet light, which is why the lights in the area where all this happens are yellow. The reason is in "Why is the lithography area in a chip fab yellow?", so it isn't repeated here.

What are the steps of photolithography?

1. Coat: a little photoresist goes onto the center of the wafer, and the wafer spins fast. The excess flies off, and what's left is a thin, even layer, about a micrometer thick or less: dozens of times thinner than a hair, or more. A short bake drives out the solvent so the film settles.
2. Expose: the scanner shines light through the mask. Where the mask lets light through, the resist changes; where it blocks the light, nothing happens. You can't see a pattern yet, just like film that has been shot but not developed.
3. Develop: first, another short bake on a hot plate (the post-exposure bake) lets the reaction inside the resist finish. Then a chemical rinses the wafer. With one kind of resist, the lit parts wash away; with the other kind it's the opposite, and only the lit parts stay. The resist left on the wafer is the pattern. This step is called develop.
4. Check: measure how wide the lines came out (line width), whether they line up with the layer below, and whether there are defects.
5. Hand off to etch: where resist covers the wafer, it's protected; where it doesn't, etch removes the material. Afterward the leftover resist is removed, and this layer's pattern is now really cut into the wafer. For what etch does, see "What do the seven process areas of a chip fab do?".

Coating, baking and developing usually happen in one coater/developer track. It's connected to the scanner, and wafers travel back and forth between the two on their own.

How does the scanner print? Like a shrinking stamp

The pattern on a photomask is usually drawn four times larger than the real thing. The scanner's lenses shrink it onto the wafer, so small errors on the mask shrink too.

The scanner prints one small square at a time, called an exposure field, roughly two to three centimeters on a side. A field may hold one chip or several. After each field, the wafer moves a little and the next one is printed, like using the same stamp to fill a sheet of paper square by square.

Because it's the same "stamp", one dirty spot on the photomask prints the same spot in the same place in every field, and many chips on every wafer can fail together. So masks are inspected regularly, and a thin, transparent protective film called a pellicle is stretched over them. Dust that lands on the film sits a few millimeters away from the pattern, out of focus, so it usually doesn't print.

Why repeat it layer after layer, dozens of times?

A chip isn't made in one go. It's more like a building: at the bottom are the transistors, the switches; above them are layer after layer of metal wiring that connects the switches, like the pipes and cables running through each floor. For more on the switches, see "What is a transistor?".

Every layer has a different shape, so for every layer a new film goes down, litho prints that layer's pattern, and etch cuts it. A simple chip has around ten layers; a complex one has dozens, each with its own photomask. Before a wafer is finished, it comes back to litho dozens of times.

Every layer has to line up: what is overlay?

Think of color printing: each color is printed separately, and if one pass is off, the color ends up outside the lines. Chips are far stricter. The wiring on one layer has to land exactly on the switches below it. Shift it a little, and things that should connect don't, while things that shouldn't touch do.

How well this layer lines up with the one below is called overlay. Before exposing, the scanner looks for alignment marks that earlier layers left on the wafer, works out how far the whole wafer is shifted and rotated, and then decides where to print each field. The allowed error is only a fraction of the finest line in the circuit. Overlay is measured on every lot; if it's off, the scanner is adjusted so the next lot comes out right.

What decides how fine it can print?

Mainly the wavelength of the light. Think of light as a pen: the shorter the wavelength, the finer the tip, and the finer the lines it can draw. So over the decades, exposure light has moved to shorter and shorter wavelengths: from violet-blue light to ultraviolet, to the deep ultraviolet that is common today, at 193 nm. With better lenses and many clever tricks, that light can print lines only a few tens of nanometers wide.

The most advanced is extreme ultraviolet, at just 13.5 nm. Even air absorbs it, and it takes a very different kind of machine; that's for another article. For what the "nanometers" in the news mean, see The news says "2 nm". What exactly is 2 nm?

Can a mistake be undone?

Yes, and that's what sets litho apart from the other steps. After develop, the pattern exists only in the resist; the material underneath hasn't been touched. If the line width is wrong, the overlay is out of spec or there's a defect, the resist is stripped off and the wafer is coated, exposed and developed again. That's called rework.

Once the wafer has gone through etch and material has been removed, there's no going back. That's why the measurement right after litho is one of the most important checkpoints on the whole line. Rework has a cost too: the wafer makes another trip and waits hours longer.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", then tap "Switch roles" to pick a litho role. As the litho engineer (production fab › process › litho), in the morning one lot's overlay is out of spec and its next step is etch: rework it, or run it as is? In the afternoon, after maintenance on the coater/developer, the line width comes out wide on one side and narrow on the other, and you pick which wafers to rework. In the evening, the after-develop check sends an image: the same spot in the same place in every field. It's the photomask. Stop the scanner and swap the mask, or mark it and let tonight's lots finish?

The same area has a litho equipment engineer, whose scanner stops in the middle of the night because it can't find the alignment marks. The mask engineer looks after the fab's photomasks: receiving them, inspecting them, their pellicles, sending them out for cleaning. On the R&D line, the litho R&D process engineer finds the focus (where the light comes to a point) and the dose (how much light) for a new layer. These roles are all marked Beta. The companies, people, tool numbers and figures in the game are fictional; what litho does, why the layers must line up and why litho can be reworked come from public, general sources. For the jargon, see Chip fab jargon in plain English.