What is thin film deposition? How chips are built up layer by layer

Thin film deposition means laying a very thin layer of new material onto a wafer. A chip is built up one layer at a time: some layers insulate, keeping electricity apart, and some conduct, carrying it where it needs to go. Most of these new layers are grown in the thin film area, and they are usually only a few nanometers to a few hundred nanometers thick. There are three common methods: chemical vapor deposition (CVD), physical vapor deposition (PVD, which in a fab mostly means sputtering) and atomic layer deposition (ALD).

Why is a chip built layer by layer?

At the very bottom of a phone chip are more than ten billion tiny switches called transistors (see "What is a transistor?"). Switches are only useful once they are wired together, so layer after layer of metal wiring goes on top, with insulating material between the wires.

Every layer follows roughly the same rhythm: lay down a layer of material, print the pattern with litho, then etch away what isn't needed. Thin film is the first step: there has to be a layer before there is anything to etch. For what the other areas do, see "What do the seven process areas of a chip fab do?".

How thin is a thin film?

Usually a few nanometers to a few hundred nanometers. A hair is tens of thousands of nanometers thick, so even a film a few hundred nanometers thick is tens to hundreds of times thinner. One layer of atoms is only a few tenths of a nanometer thick, and the thinnest films are just a few nanometers: you could count the layers of atoms.

Insulating films are often silicon dioxide (the main ingredient of glass) or silicon nitride; conducting films are mostly metals.

People say a film is "grown" either way, but there are two kinds. Oxidation in a furnace makes the silicon itself react with oxygen, so the layer grows out of the wafer. The thin film area brings the material in from outside and lays it on top.

What is chemical vapor deposition (CVD)?

CVD sends a few gases into a sealed chamber, where they react on the hot wafer surface and leave a solid layer behind; the leftover gas is pumped away. For example, a gas that contains silicon plus a gas that contains oxygen leaves a layer of silicon dioxide.

It is a bit like a window in winter: moisture touches the cold glass and turns into a layer of frost. The difference is that frost is just water vapor turning into ice, while in CVD the gases react on the surface and become something else.

The gases come down evenly through a showerhead full of small holes. Some CVD gets help from a plasma, so the wafer doesn't need to be as hot; later in the flow the wafer already has metal wiring and can't take much more heat (the thermal budget). Gases get into holes and react on every surface, so CVD is better at coating the sidewalls and bottoms of holes.

What is physical vapor deposition (PVD), or sputtering?

PVD doesn't rely on a chemical reaction. The kind fabs use most is sputtering: the chamber is pumped down to a vacuum, and a little argon is let in and lit into a plasma. The charged argon is pulled by an electric field into a plate of material (the target, usually a metal) and knocks atoms off it one by one. They land on the wafer as a film.

It needs a vacuum because in air an atom can't travel a ten-thousandth of a millimeter before it hits an air molecule, and oxygen and moisture would get into the film.

It works like spray paint: the paint flies out in straight lines, and whatever faces the nozzle gets the thickest coat. In a deep, narrow hole, most atoms land on top and few get inside, so the bottom and sidewalls stay thin. The target keeps getting hit, so it wears thinner and eventually has to be replaced.

What is atomic layer deposition (ALD)?

ALD splits deposition into many rounds, and each round adds at most one layer of atoms, often less.

Picture a parking lot where each space fits one car: once it's full, any car that arrives has to drive away. ALD first lets in one gas, whose molecules "park" on the wafer surface until one layer is full, and then no more can stick. The extra gas is flushed out, and a second gas comes in and reacts with the parked layer, turning it into a layer of film and opening up new spaces on top. Flush again, and the next round begins.

Because each space holds just one car, the film comes out the same thickness at the mouth of a hole and deep at its bottom, as long as there is enough time. The price is speed: in one common process, each round adds about 0.1 nanometer, so 10 nanometers takes about a hundred rounds. That's why ALD mostly goes where films are thinnest and matter most, such as an insulating layer in a transistor that is only a few nanometers thick.

How are CVD, PVD and ALD different?

Method	How it grows	Like	Good at	Weak spot
CVD (chemical vapor deposition)	Gases react on a hot surface	Frost on a cold window	Fast, many materials, reaches into holes	In very deep, narrow holes, the opening can close first
PVD (physical vapor deposition, sputtering)	Atoms knocked off a target	Spray paint	Metal films, fast	Thin at hole bottoms and sidewalls
ALD (atomic layer deposition)	Two gases take turns, at most one layer per round	A parking lot filling up	The most even; handles very thin films	Very slow

The thing to fear most is a hole's opening closing first. Transistors connect to the wiring above through small, deep holes (contacts) that have to be filled with metal. If the opening closes first, a void is left inside: it looks fine from above, but current doesn't flow well. The finer and deeper the holes, the more the method and the thickness matter, and the three methods are often used together.

Why does the thickness have to be even?

Thickness decides how a film behaves: a thinner metal film has higher resistance, and a thinner insulating film may not insulate. Etching doesn't like uneven films either: on one wafer, the thin spots break through first while the thick ones haven't yet, so the etch has to run a while longer; the bigger the difference, the longer it runs, and the more the layer under the thin spots can get damaged. So the average isn't enough; the center and the edge of each wafer need to match.

So films get measured. For a transparent film, the reflected light gives the thickness; metal doesn't let light through, so how well it conducts is measured instead (sheet resistance). Many points are measured on one wafer and drawn as a thickness map, and the tool runs a monitor wafer on a schedule to catch slow drift.

The right thickness still isn't the whole story: a film pulls or pushes on the wafer and bends it slightly. Bend it too much, and litho can't get it in focus.

Why can't there be particles in a film?

During deposition, a single particle landing on the wafer gets buried under the film and leaves a small bump. The layers above can go crooked too, and a line can break or two lines can touch. For how clean a cleanroom is, see "How clean is a cleanroom?".

Particles can also come from the tool itself: the film grows on the chamber walls and parts too, and once it builds up, it flakes off. So chambers are cleaned on a schedule, and after maintenance, test wafers are run to count particles (a qual run) before lots go back in.

Why polish it flat afterwards?

A film follows the shape of what's underneath: bumps below mean bumps on top, and filling trenches with metal leaves an extra layer on the surface. So thin film is often followed by CMP, which polishes the surface flat and removes the extra metal, so the next litho step can line up (how CMP works: "What do the seven process areas of a chip fab do?"). A film a little thicker or thinner changes what comes out of polishing, so the two areas often have to talk.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", and pick a thin film role right on the "Your role today" card. As the thin film process engineer (production fab › process › thin film), you look after PVD-01, a sputtering tool. Late at night, the film thickness passes but the wafers bend more than usual. In the morning, the target is nearly used up and the film gets thinner every day. In the afternoon, a new target goes in, particles go up, the film comes out thicker, and CMP, the next step, calls. The evening is the new target's first night.

The same tool's vacuum, pumps and target changes belong to the thin film equipment engineer (production fab › equipment › thin film). On the R&D line, the thin film R&D process engineer uses CVD-R1 to put the barrier layer and the metal into the new contacts, and a pre-dawn cross-section photo shows a void at the bottom of a hole. These roles are marked Beta. The game has no ALD tool. The companies, people, tool numbers and figures are fictional; how films are grown, and why unevenness and particles matter, follow public, general sources. For other jargon, see Chip fab jargon in plain English.