What Is a Cleanroom? Why Chip Fabs Fight Dust
A cleanroom is a room where invisible particles in the air (dust) are kept extremely low. Chip circuits are so fine that a single tiny particle can make a chip fail. That is why the steps that build circuits on a wafer happen in this room. Filtered air flows from the ceiling to the floor nonstop, and dirt from people and chemicals is tightly controlled.
What you'll learn
- Why one tiny particle can cause a big loss
- How to read cleanliness levels such as "ISO Class 5" and "Class 100"
- How filters, airflow and air pressure keep the room clean
- Why people are the biggest source of dirt, and why they wear full-body suits
- How sealed wafer boxes take the pressure off the whole room
- Dirt other than particles (metals, gases, static electricity), plus temperature, vibration and power use
Prerequisites: none. It helps to know that chips are made on a round plate called a wafer by adding films, printing patterns and etching, over and over. "How Are Chips Made?" covers the whole process.
In everyday life
News stories about new chip factories often show photos of people covered in white from head to toe. They wear masks and gloves, and only the area around their eyes shows. Sometimes the room behind them glows yellow. This room is a cleanroom.
The yellow rooms are where circuit patterns are printed with light. The coating used for printing patterns reacts to blue light and ultraviolet light. The room lights filter out blue and ultraviolet light, so the room looks yellow.
This article looks at how clean a cleanroom is, how it stays clean, and why it has to be so clean.
Why one tiny particle is a problem
The finest wires on today's chips are less than one-thousandth of the width of a human hair (about 0.09 mm). If a particle lands on them, it can join two neighboring wires or break a wire in the middle. A flaw like this that makes a chip fail is called a killer defect.
A particle does not choose which chip it lands on. One wafer holds hundreds to thousands of chips, so more falling particles means more bad chips. On top of that, chips are made by repeating the pattern-printing step more than 100 times. Each step is another chance for a particle to fall, so even small odds add up.
The share of good chips among all the chips made is called the yield. Processing one wafer costs about the same no matter how many good chips come out. For example, if good chips drop from 90% to 80%, the cost per good chip rises about 1.1 times (0.9 ÷ 0.8). You can try how chip size and particle count change the number of good chips with the die per wafer and yield calculator.
How clean? Cleanroom "classes"
Cleanliness is measured by counting the particles in the air. The international standard ISO 14644-1 sorts air cleanliness into "classes" from 1 to 9. Each class sets how many particles of a given size or larger a fixed amount of air may hold. The smaller the number, the cleaner the air.
Particle sizes are given in µm (micrometers). One µm is one-thousandth of a millimeter, or 1,000 nm. The amount of air is 1 m³, a cube 1 m on each side.
The basic rule is simple. The limit for particles of 0.1 µm or larger is 10 in Class 1 and 100 in Class 2. Each step up in class number allows 10 times as many. Fewer large particles are allowed than small ones. Class 5 allows up to 100,000 particles of 0.1 µm or larger, but only 3,520 of 0.5 µm or larger.
| ISO class | Particles ≥ 0.1 µm | Particles ≥ 0.5 µm | Rough guide |
|---|---|---|---|
| 1 | 10 | — | Cleanest |
| 3 | 1,000 | 35 | |
| 5 | 100,000 | 3,520 | Old "Class 100" |
| 6 | 1,000,000 | 35,200 | Old "Class 1,000" |
| 7 | — | 352,000 | Old "Class 10,000" |
| 9 | — | 35,200,000 | Close to city outdoor air |
The numbers are the maximum allowed per 1 m³ of air. A dash (—) means the standard's table sets no limit there. Classes 2, 4 and 8 are left out.
According to the University of Kentucky, city outdoor air can hold as many particles as Class 9. Compare the limits for particles of 0.5 µm or larger, and Class 9 allows 10,000 times more than Class 5. Per liter, that is about 35,000 particles for Class 9 and about 3.5 for Class 5.
In fabs and in the news, you still hear old names such as "Class 100." They come from a US standard, Federal Standard 209E, which was canceled in 2001. The number is the count of particles of 0.5 µm or larger in one cubic foot (about 28 liters) of air. Class 100 roughly matches ISO Class 5.
Step 1: Filters catch the particles
Filters lined up in the ceiling remove particles from the air. They are made of very thin fibers, such as glass fibers, layered like thin paper and folded into pleats. They do more than sieve out particles bigger than the gaps. Particles carried by the airflow hit or brush against the fibers, then stick and stay there.
A common type is the HEPA filter. It catches 99.97% or more of 0.3 µm particles, under Japan's JIS standard and the US Department of Energy standard. That means at most 3 out of 10,000 particles get through. 0.3 µm is close to the size that fibers find hardest to catch. Larger particles, and much smaller ones, are caught even better.
A higher-performance type is the ULPA filter. Japan's JIS standard defines it as catching 99.9995% or more of 0.15 µm particles. US and other standards use slightly different test particle sizes and percentages. Under the JIS definition, at most 5 in a million particles get through. "HEPA vs ULPA filter" compares the two in detail.
Both filters catch only particles, though. Gas molecules, like the ones behind smells, pass right through. How fabs deal with gases is covered later, under "Dirt that isn't particles."
Step 2: Keep the air flowing downward
Clean air from the ceiling filters is sent straight down. This way of moving air is called downflow. Particles from people and machines are pushed toward the floor before they can spread sideways.
The floor is made of panels full of holes, with an open space underneath. Air passes through the holes into this space, travels up ducts along the walls, and returns above the ceiling. Then it goes through the filters into the room again. Only part of the air comes from outside. Most of it is the same air, filtered again and again.
A typical air speed is about 0.3–0.5 m/s. That is a gentle breeze, slower than a slow walk. Even so, with a 3 m ceiling, the air takes only 6–10 seconds to reach the floor (a rough calculation).
One more trick is to keep the room's air pressure slightly higher than its surroundings. Then, when a door opens or air finds a gap, it flows from the inside out. Dirty outside air has a hard time getting in.
The biggest source of dirt: people
Inside a cleanroom, people are the ones who keep shedding particles. Skin flakes off little by little, and clothes shed bits of fiber. Every time a person moves, these get stirred up into the air.
A 2007 study by Whyte and Hejab in the UK measured the particles given off by 55 people. In ordinary clothes, people gave off about 2.1 million particles of 0.5 µm or larger per minute on average. Even in the study's cleanroom clothing, the figure was about 1 million. The numbers vary a lot with clothing and measuring method. Still, they show that clothing cannot bring the particles from people down to zero.
So workers wear special garments that cover them from head to toe. They also wear a hood, a mask, gloves and special shoes. The garment is made of fabric that sheds very little. Its job is to trap the particles from the person inside the suit. These suits are often nicknamed "bunny suits."
After suiting up, workers pass through a small room that blasts strong air over the whole body. This is called an air shower. It blows particles off the surface of the suit, so fewer get carried into the room. Avoiding sudden movements inside the room is another important rule for keeping particles down.
Protect only the space around the wafer
Older fabs protected wafers by keeping the whole room extremely clean. But filtering the air of a large room nonstop takes a lot of money and electricity. And as long as wafers share the air with people, particles from people are hard to avoid.
So today's fabs carry wafers in sealed boxes. One box holds up to 25 wafers of 300 mm diameter, and it is called a FOUP. A full one is heavy, so carts running on ceiling rails move it from tool to tool automatically.
At the front of each tool is a small room that opens the FOUP's lid and takes the wafers out. It has its own filters, and the air inside is kept very clean. This idea of keeping only the small space around the wafer especially clean is called a mini-environment.
Wafers never touch the air of the room where people work directly. That is why the standard for the whole room could be relaxed. A 2007 article in Hitachi Hyoron, a Japanese technical journal from Hitachi, points the same way. Push cleanliness high mainly inside the mini-environments. Keep the whole room only as clean as tool maintenance requires. This saves money and electricity.
Dirt that isn't particles
Cleanrooms guard against more than invisible particles. Here are three main examples.
- Metals: Human sweat and skin contain sodium. If sodium gets into the insulating film of a transistor, it moves around inside the film. This shifts the voltage at which the transistor starts to conduct. Charged particles that move like this are called mobile ions, and they are strictly controlled.
- Gases in the air: Some pattern-printing coatings react using a tiny amount of acid made by light. If ammonia in the air cancels out this acid, the pattern loses its shape. Dirt carried by gases like this is called airborne molecular contamination (AMC). Special filters, using materials such as activated carbon, remove it.
- Static electricity: Objects charged with static electricity pull in particles from the air. A sudden discharge of static can even destroy a chip's circuits (electrostatic discharge). So floors and workbenches use materials that let charge drain away, and devices that cancel charge are installed too.
The water used to wash wafers cannot be ordinary water either. Fabs use ultrapure water, with metals, ions, particles and organic matter removed as far as possible. Its purity is checked by how poorly it conducts electricity (its resistivity). Water itself splits slightly into ions, so even perfectly pure water has an upper limit on resistivity. At 25 °C, the limit is 18.2 MΩ·cm. Ultrapure water comes extremely close to this limit. For how wafers are washed, see the comparison page "SC-1 vs SC-2 vs dilute HF."
Temperature, humidity and vibration are controlled too
Things grow slightly when they warm up. When silicon warms by 1 °C, it grows by about 2.6 millionths of its length. For a 300 mm wafer, that works out to about 0.78 µm (780 nm). When patterns a few tens of nm wide must line up exactly, that is too big to ignore. So the temperature is held steady, especially around the tools that print patterns.
Humidity is held steady too. Air that is too dry makes static electricity more likely. Moisture also affects how the pattern-printing coating behaves.
Vibration is another enemy. Pattern-printing tools align their positions to nanometer accuracy. Even slight shaking from nearby machines or people can throw the position off. Fabs are built so that tools sit on sturdy floors and foundations that do not pass on vibration easily.
Staying clean takes electricity
In a cleanroom, large fans keep the air moving day and night. The air's temperature and humidity are also kept the same all year. Researchers at Lawrence Berkeley National Laboratory in the US gave an estimate in a 2000 report. Cleanrooms and laboratory facilities use roughly 4 to 100 times the energy of ordinary commercial buildings of the same floor area.
Relaxing the room standard with mini-environments is partly a way to cut this electricity and cost. News about new fabs often mentions securing power and water. One reason is that these systems must run without a break.
Common misconceptions
Myth: Everywhere inside a fab is equally, extremely clean
In fact, different areas need different classes. In today's fabs, the cleanest spaces are the small ones around the wafer, such as inside FOUPs and tools. The rooms where people work are kept to a looser standard. Even so, "looser" still means far cleaner than outdoor air. The myth comes from fab photos, where every room and every suit looks the same.
Myth: A cleanroom is a "sterile room," like a hospital operating room
In fact, the goal is different. ISO 14644-1 classes depend only on the number of particles. They do not care whether a particle is a living germ. In operating rooms and drug factories, reducing germs is a major goal. Chip fabs care most about particle counts and chemical dirt such as metals and gases. The word "clean" suggests hygiene, which is where this myth comes from.
Myth: The people in pure white suits are the cleanest things in the room
In fact, people are the biggest source of dirt in the room. The suits are worn less to protect the people and more to trap the particles they give off. Even in a suit, the more a person moves, the more particles leak out. The myth comes from how white and clean the suits look.
By the numbers
| Number | What it means |
|---|---|
| 10 | Particles of 0.1 µm or larger allowed per 1 m³ of air in ISO Class 1 |
| 3,520 | Particles of 0.5 µm or larger allowed per 1 m³ of air in ISO Class 5 (the old Class 100) |
| 10,000 times | Gap in the limit for particles of 0.5 µm or larger between Class 9 (close to city outdoor air) and Class 5 |
| 99.97% or more | Share of 0.3 µm particles a HEPA filter catches (JIS and US Department of Energy standards) |
| 0.3–0.5 m/s | Typical speed of the air flowing from ceiling to floor |
| About 1 million | Particles of 0.5 µm or larger given off per minute by a person in cleanroom clothing (Whyte et al., 2007) |
| 25 | Maximum number of 300 mm wafers one FOUP holds |
| 18.2 MΩ·cm | Upper limit of water's resistivity at 25 °C. Ultrapure water comes close to it |
| About 0.78 µm | How much a 300 mm silicon wafer grows with a 1 °C rise (calculated) |
| About 4–100 times | Energy per floor area of cleanrooms and laboratory facilities compared with ordinary commercial buildings (2000 report) |
Put the numbers side by side, and the aim of a cleanroom becomes clear. It makes air more than 10,000 times cleaner than outdoor air, all year without a break. Even so, it cannot fully shut out people, the biggest source of dirt. So fabs also seal wafers inside boxes and tools to protect them.
Learn more
- Earlier in this series: "What Is a Silicon Wafer?" (making particle-free wafers), "What Is EUV Lithography?" (keeping dust off masks), "What Is Semiconductor Packaging?"
- From the equipment side: "Semiconductor Manufacturing Equipment" introduces cleaning tools and the inspection tools that hunt for particles.
- Go deeper with terms: the FFU (the filter box in the ceiling), the EFEM (the transfer area at the front of a tool), and the OHT (the system that carries FOUPs), among others.