Plasma in Semiconductor Manufacturing: Etching and Sheaths

In everyday life

You may have seen the word plasma in a science textbook or in ads for home appliances. In fact, glowing plasmas are all around us.

• Fluorescent lamps: The tube holds a little mercury vapor and some gas. Electrons from the electrodes hit mercury atoms, which give off invisible ultraviolet light. A phosphor coating on the inside of the tube turns it into white light. The gas inside the tube, full of flying electrons, is a plasma.
• Neon signs: Run electricity through neon gas sealed in a tube, and it glows reddish orange. Other colors come from other gases or from coatings inside the tube.
• Lightning: A huge electric current flows through the air in an instant, and the air along its path becomes a plasma. According to the US National Weather Service, that air reaches about 28,000 °C (50,000 °F).
• The aurora: Electrons come down from space along Earth's magnetic field. According to the US National Oceanic and Atmospheric Administration (NOAA), they hit oxygen and nitrogen about 80–500 km up, which then glow. These electrons come from the plasma in the space around Earth.
• The Sun: The Sun is a giant ball made entirely of plasma. According to NASA, even its visible surface is about 5,500 °C.

All of these glow. But not all of them are hot. A fluorescent tube never gets too hot to touch. These two points are clues to understanding the plasma used in fabs.

What is plasma? A gas that has lost electrons

An atom is made of a positively charged nucleus and negatively charged electrons around it. Normally the plus and minus charges balance, so the atom as a whole has no charge. A gas is a state in which such atoms or molecules fly around separately.

Give a gas enough energy, and some atoms or molecules lose an electron. What is left behind carries a positive charge. This charged particle is called an "ion."

Now part of the gas has split into free electrons and positive ions, all flying around. This state is called plasma. The negative electrons and positive ions come in almost equal numbers, so the whole thing has no overall charge.

In a gas of molecules, which are several atoms joined together, one more thing happens. An electron can hit a molecule and break it into fragments. The fragments carry no charge, but they are very reactive. They quickly bond with whatever they meet. Such a fragment is called a radical.

A plasma glows because atoms and molecules struck by electrons release the energy they received as light. The type of gas decides the color of that light. That is why neon glows orange.

What "the fourth state of matter" means

Heat ice and it becomes water. Heat it more and it becomes steam. These three forms are called solid, liquid and gas. Give a gas even more energy, and electrons break free to form a plasma. So plasma is also called "the fourth state of matter."

The name goes back to the 19th-century British scientist William Crookes. He called the very thin gas inside a discharge tube a "fourth state." The word "plasma" was first used by the American scientist Irving Langmuir in a 1928 paper.

However, this is a simplification to make things easy to grasp. There is no sharp boundary like ice melting at 0 °C. The share of particles that have lost electrons simply rises little by little. As we will see, most of a fab plasma is still ordinary molecules.

How do fabs make plasma?

First, a pump removes the air from a metal chamber that holds the wafer. Then a small amount of process gas flows in. In an etching tool, the pressure inside is roughly 0.1 to a few tens of Pa (pascals). One pascal is about 1/100,000 of the air pressure at ground level.

Next, the chamber is driven with high-frequency power. This is electricity whose direction flips millions of times per second. Many tools use 13.56 MHz (13.56 million cycles per second), a frequency set aside for industrial, scientific and medical use. At this frequency, the rules on radio waves leaking out are relatively relaxed.

The electric field speeds up the few electrons already in the gas. A fast electron hits a molecule. It may knock a new electron loose, or split the molecule into radicals. The freed electron is also accelerated and hits the next molecule. Some electrons are lost when they hit the walls. The plasma holds steady when the electrons gained and lost balance out.

Surprisingly, very few molecules actually lose an electron. Gas at 1 Pa and room temperature holds about 240 trillion molecules per cm³. Suppose a high-density tool has 100 billion electrons per cm³. Dividing one by the other, only about 4 in 10,000 molecules have lost an electron (a rough estimate from this calculation).

There is also a reason for the low pressure. With fewer molecules around, an electron can travel farther before a collision, so it can pick up more speed. Ions, too, can fly straight without hitting other molecules on the way.

Hot electrons, lukewarm gas

The "temperature" of particles is the average energy of their motion. In a plasma, the electrons and the other particles have very different temperatures. In a low-pressure processing plasma, the electron temperature is about 20,000 to 60,000 degrees. The ions and gas, meanwhile, stay near room temperature, at most a few hundred degrees.

The reason is that electrons are very light. An electron weighs only about 1/1,840 as much as a hydrogen atom, the lightest atom. When a light particle hits a heavy one, it just bounces off and barely moves the heavy one. So only the electrons, pushed by the electric field, build up energy. The gas as a whole does not get hot.

Also, electrons are far fewer than molecules, so all the electrons together carry very little heat. That is why the electron temperature does not melt the wafer. The stage holding the wafer is temperature-controlled, and it removes the heat added by ion impacts.

This property is a big reason plasma suits chipmaking. Some chemical reactions normally need heating to several hundred °C or more. In a plasma, fast electrons break the molecules apart instead, so these reactions run at low temperature. Some materials on a chip cannot take much heat, so this matters a lot.

The plasma sheath above the wafer

A special thin layer forms where the plasma meets the chamber walls or the wafer. Electrons are light and fast, so they reach the walls and the wafer before the heavy ions do. As a result, the wafer surface becomes negative compared with the plasma.

The negative surface pushes back further electrons and pulls in positive ions. So, just above the surface, a thin layer forms that pulls ions strongly toward it. This layer is called the "sheath," like the cover around a blade. It has few electrons, so it glows only faintly. It looks like a dark gap between the plasma and the wafer.

Ions that enter the sheath are accelerated straight toward the surface. The wafer is flat, so the ions rain down almost vertically. In an etching tool, the stage holding the wafer also gets high-frequency power. The stage then becomes even more negative, and the power level sets how hard the ions hit. The ions are often accelerated through tens to hundreds of volts.

Use 1: plasma etching along the pattern

In chipmaking, a light-printed pattern is left in a layer of light-sensitive material on top of a film. This material is called photoresist, or resist for short. Next, the film is etched away wherever the resist does not cover it.

Etching with plasma is called dry etching. It uses gas instead of liquid chemicals, hence the word "dry." Here, radicals and ions split the work.

Radicals bond with atoms of the film and form substances that turn into gas. For example, fluorine radicals bond with silicon to form gaseous silicon fluoride, which is pumped away. But radicals arrive from every direction. On their own, they etch down and sideways at the same rate.

Ions, accelerated in the sheath, strike from straight above. Where an ion hits, the surface is disturbed, and its reaction with radicals speeds up sharply. A famous experiment by Coburn and Winters at IBM (1979) showed this. With a fluorine-releasing gas and ions together, silicon etched about 10 times faster than the two acting separately, added together.

Ions mostly hit the flat bottom of a trench. They barely touch its vertical sidewalls. So the bottom etches fast while the sides etch only slowly, giving straight, vertical trenches and holes. Etching that runs at different speeds in different directions is called anisotropic etching.

The opposite kind etches sideways just as fast. You can compare the cross-section shapes in "Isotropic vs Anisotropic Etching." The classic method that combines radicals and ions is called reactive ion etching (RIE). Real tools use further tricks, such as coating the sidewalls with a thin protective film while etching. We skip those here.

Use 2: depositing films at low temperature

Plasma is also used to add films. One method reacts source gases so that a film builds up on the wafer. It is called CVD (chemical vapor deposition). Using heat alone, a silicon nitride film needs about 700–800 °C.

With plasma, electrons break up the source gases, so the film can be deposited at about 300–400 °C. This method is called plasma-enhanced CVD (PECVD). Aluminum wiring melts at about 660 °C. So after the wiring is in place, low-temperature deposition is a must. The trade-off is that the film keeps more hydrogen and is less dense than a heat-only film. "LPCVD vs PECVD" compares the two in a table.

Metal films are often made by sputtering. A plasma is made from argon gas, and its ions are fired at a metal plate. Metal atoms knocked out of the plate land on the wafer and build up a film. This method does not rely on chemical reactions. It uses the momentum of ions accelerated in the sheath.

Use 3: removing and cleaning

Once etching is done, the resist used to print the pattern is no longer needed. Resist is an organic material made of carbon and hydrogen. Oxygen radicals from an oxygen plasma turn it into carbon dioxide and water, which leave as gas. This step is called ashing, from the idea of burning something to ash.

Ashing often uses tools that make the plasma away from the wafer. Ions recombine with electrons and vanish before they arrive, so only the long-lived radicals reach the wafer. This keeps ion impacts from damaging the circuits. The same idea is used to gently remove traces of dirt or thin films from a surface.

Here are the uses, sorted by the particles that do most of the work.

Common misconceptions

By the numbers

In short, a fab drives just a tiny fraction of the electrons in a thin gas to tens of thousands of degrees. It keeps the gas itself cool and uses the reactive particles this creates to process the wafer.

Learn more

• In this series: "How Are Chips Made?" (the whole process), "What Is EUV Lithography?" (making light with a hot tin plasma), "What Is a Cleanroom?"
• From the equipment side: "Semiconductor Manufacturing Equipment" introduces the makers of etching and deposition tools.
• Compare in a table: "Wet vs Dry Etching," ways of making plasma in "CCP vs ICP Plasma," and ways of adding films in "CVD vs PVD."
• Go deeper with terms: ICP (high-density plasma), self-bias (the voltage on the wafer), selectivity (the ratio of etch rates) and atomic layer etching (one atomic layer at a time).