What is plasma, and why do chip fabs etch with it?

Plasma is a gas that electrical energy has "pulled apart": some molecules are split into charged ions and electrons, and many more become very reactive fragments that are eager to react with something, all mixed with the gas that is still intact. A chip fab makes plasma by feeding gas into a chamber pumped down to low pressure and adding high-frequency electric power. Etching uses it because it does two jobs at once: the fragments handle the chemistry, and the charged ions are pulled down by an electric field to do the hitting. So the etch goes only downward, cutting narrow trenches with straight walls.

How dry and wet etching differ is covered in "What is etching? How are dry and wet etching different?". This article goes one layer deeper: what plasma is, and what an etch tool does, step by step.

What is plasma? Can you see it in everyday life?

Heat ice and it becomes water; heat water and it becomes steam. Give a gas even more energy and some molecules break into fragments, while others lose electrons and become positively charged "ions." That is plasma, often called the "fourth state of matter." The gas inside a lit neon sign or fluorescent tube is plasma, and so are lightning and the auroras. Different gases glow in different colors.

In a fab's plasma, only a small share of the gas actually becomes ions, often less than one percent. The electrons move extremely fast, which works out to a temperature of tens of thousands of degrees. But electrons are very light, and the gas itself isn't hot. So reactions can happen without baking the wafer at a high temperature.

How does a fab make plasma?

• A low-pressure chamber: pumped down to roughly one ten-thousandth to one hundred-thousandth of atmospheric pressure, or even lower. In thin gas, an electron has room to speed up before it hits the next molecule, so it more easily builds up enough energy to break molecules apart.
• Gas: chosen to match the material being etched; gases containing fluorine or chlorine are common.
• High-frequency power: the current swings back and forth more than ten million times a second (13.56 MHz is common), pushing energy into the gas.

Many etch tools have two power supplies. The one on top lights the plasma and sets how dense it is; the one under the wafer sets how hard the ions hit.

Why does etching use plasma?

• Fragments do the chemistry: textbooks call them "radicals." They react with the exposed material, turning it into a gas that the pump carries away.
• Ions hit straight down: electrons are light and fast, so they reach the wafer first and give its surface a negative charge, which draws in the positively charged ions. In a very thin layer just above the wafer, the electric field points straight at the wafer, and the ions come down almost vertically.

In a well-known experiment, etching silicon with only ions or only a reactive gas was slow, but using both at once was several times faster than either alone. Ions only reach what is directly below them, so the bottom of a hole keeps getting etched while the side walls barely change.

Why don't the side walls get eaten away?

The fragments have no direction, so they could wander into the side walls too. That's why many etches add ingredients to the gas that "grow a film," laying a thin protective coating on the surfaces. At the bottom of a hole, ions keep knocking the film off, so etching continues downward; the side walls are rarely hit, so the film stays and blocks the fragments. Too little film and the walls get eaten; too much and the hole narrows as it goes. With gases containing carbon and fluorine, for example, adding a bit more oxygen makes the film easier to remove; that's how the mix is tuned.

How does a wafer go through an etch tool?

Step 1: Into the chamber, held down, helium on the back

• What: a robot arm carries the wafer into the chamber and sets it on an electrostatic chuck, which holds it flat; helium flows into the thin gap between the back of the wafer and the chuck (helium backside pressure).
• Why: in a vacuum, suction can't hold the wafer, so static electricity does it. A vacuum also barely conducts heat, like a thermos, so the heat from the plasma has to travel through the helium to the cooled base below.
• Otherwise: the wafer gets hotter as it etches, and the shape and line width of what's etched drift.

Step 2: Close the door and pump down

• What: the door closes, and the chamber is pumped to an even lower pressure.
• Why: only the gases about to be fed in should be inside.
• Otherwise: stray gas and moisture get into the reaction, and the result no longer matches the settings.

Step 3: Feed in the gases and hold the pressure

• What: following the recipe (the settings the tool runs), several gases flow in, each measured in sccm, and the pressure is held at its set value.
• Why: the gases and the pressure decide what gets etched and how straight (see the table below).
• Otherwise: if the flow or pressure wanders, every wafer comes out different.

Step 4: Light the plasma

• What: the top power supply switches on and a glow fills the chamber; then the supply under the wafer switches on to pull the ions down.
• Why: one controls how dense the plasma is and the other how hard the ions hit, and keeping them separate lets each be set precisely.
• Otherwise: with only one supply, the two are tied together, and it's hard to get both just right.

Step 5: Etch

• What: fragments and ions turn the exposed material into gas and carry it away, while a protective film builds on the side walls; whatever the photoresist covers stays.
• Why: that's how the pattern is copied straight down from the resist.
• Otherwise: etch too long and the resist, which is slowly being etched too, runs out first, and areas that should stay get etched as well.

Step 6: Watch for the endpoint

• What: through a small window in the side of the chamber, the tool keeps watching the plasma's light. When this layer is etched through and the layer below shows, the color or brightness of the light changes, and the tool knows it's there. It then etches a little longer (over-etch) so that every hole reaches the bottom.
• Why: film thickness and etch speed vary a little from wafer to wafer, so watching the light is more accurate than watching the clock.
• Otherwise: stop too early and some holes don't go through; stop too late and the layer below is damaged. When the etch only digs to a certain depth inside one material, the light doesn't change, so the time is often worked out from a measured etch speed.

Step 7: Plasma off, pump out the gas, take it out

• What: the plasma is switched off and the gas pumped out, the chuck lets go, and the wafer is lifted and carried out.
• Why: many of the specialty gases used in etching are toxic or corrosive, and the exhaust passes through exhaust treatment before it's released.
• Otherwise: open the door before the gas is pumped out, and it flows into the space where wafers are handed over, reaching the next wafer and the tool's parts.

What do pressure, power and gas each change?

What you adjust	Roughly what happens
Pressure	Lower: ions are knocked off course less, so the etch is straighter. Higher: chemistry plays a bigger part, and the etch eats sideways more
Top power supply	Higher: denser plasma, faster etching
Power supply under the wafer	Higher: ions hit harder, straighter and faster, but the etch is less picky about materials, and more resist is lost
Type and mix of gases	Decides what reacts, how picky the etch is, and how much side-wall film grows

These settings are chosen together as one set and written into the recipe (see "What is a recipe in semiconductor manufacturing?").

Why do dense and sparse patterns etch differently?

• Large crowded areas to etch: the fragments get used up fast there, so there aren't enough to go around, and etching slows.
• Narrow, deep holes: fragments and ions have a harder time reaching the bottom, so the deeper it gets, the slower it goes; on the same wafer, narrow holes often reach the bottom later than wide ones.

That's why, even after the endpoint is detected, there's still an over-etch.

What happens when the backside cooling goes wrong?

If the chuck's surface is worn or there are particles on the back of the wafer, the wafer can't sit flat against it, and helium leaks out around the edge. The tool watches the helium flow: the higher the flow, the bigger the leak. A small leak and the wafer runs warm, and the line width may drift to one side, bit by bit; a big leak and the tool raises an alarm and stops, and a wafer may be stuck in the chamber halfway through its etch. So helium flow creeping up is a warning sign someone should be watching.

Is plasma used only for etching?

• Growing films: plasma-enhanced chemical vapor deposition (PECVD). The fragments are already eager to react, so the film can grow without heating the wafer as much (see "What is thin film deposition?").
• Removing photoresist (ashing): photoresist is mostly carbon and hydrogen. Oxygen lit into a plasma reacts with it to form carbon dioxide and water vapor, which are pumped away, while the silicon and oxide below are barely touched. It's like burning the resist to ash.

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 "Etch" under the name. The ETCH-03 you look after is a plasma etcher. Tap it, choose "Take it apart" and then "See it work", and it runs step by step, from the moment the wafer comes in until it goes out. You can also open the plasma etch chamber directly.

In "The life of a chip", chapter 6, Chamber A of ETCH-03 throws a helium backside pressure alarm at 02:47. In chapter 7, "Firefighting", you look for clues at the tool: the helium flow has been creeping up for three days, and the ESC is at 96% of its replacement cycle. Once the ESC is replaced, you do the qual run, deciding how many season wafers to run and how long to pump down and bake. The etch R&D process engineer on the R&D line meets ETCH-R1 down on the morning shift: the plasma's power isn't getting into the chamber and is bouncing back. The evening shift is chapter 5, "The Last Box", where the limit for the helium flow alarm has to be set.

The companies, people, tool IDs and numbers in the game are fictional; what plasma is and how etch tools use it come from public, general sources. For other jargon, see Chip fab jargon in plain English.