How clean is a cleanroom? What Class 100 and ISO 5 mean

A cleanroom's class counts the dust in its air: the most particles above a given size allowed in one cubic meter of air. The names you often hear, "Class 100" and "Class 10", come from an old US standard and mean no more than 100 or 10 particles of 0.5 micrometers or larger per cubic foot. Today the international standard is ISO 14644-1, with nine classes from ISO 1 to ISO 9: Class 100 is roughly ISO 5, and Class 10 is roughly ISO 4. Either way, the smaller the number, the cleaner the air.

What are the "particles" being counted?

A particle here is any tiny speck floating in the air: dust, skin flakes, fibers from clothes, smoke.

The size cleanrooms count most often is 0.5 micrometers. A micrometer is a thousandth of a millimeter. A human hair is a few tens of micrometers thick, so a 0.5-micrometer particle is about a hundred times thinner, far too small to see.

You may have seen PM2.5 in an air quality app. That means particles of 2.5 micrometers and smaller, measured by how much they weigh in a cubic meter of air. Cleanrooms care about even smaller ones, and they don't weigh them. They count them, one by one.

How? A particle counter draws in a fixed volume of air and sends it through a laser beam. Each particle that crosses the beam scatters a little flash of light. The number of flashes is the number of particles, and how bright each flash is tells you how big it was.

What's the difference between Class 100 and ISO 5?

"Class 100" comes from a US federal standard that measured in cubic feet. Class 100 means no more than 100 particles of 0.5 micrometers or larger per cubic foot; Class 10 means 10, and Class 1,000 means 1,000. The name is the limit. ISO 14644-1 replaced that standard in 2001, but the old names are easy to remember and people still use them.

ISO uses cubic meters. A cubic meter is about 35.3 cubic feet, so 100 per cubic foot works out to about 3,530 per cubic meter, very close to ISO 5's limit of 3,520. The two standards don't measure in exactly the same way, so people only say the classes are "roughly equal".

ISO class	Max particles ≥ 0.5 µm per m³	Old name
ISO 3	35	Class 1
ISO 4	352	Class 10
ISO 5	3,520	Class 100
ISO 6	35,200	Class 1,000
ISO 7	352,000	Class 10,000
ISO 8	3,520,000	Class 100,000
ISO 9	35,200,000	None

The number after "ISO" is really a count of zeros. For particles of 0.1 micrometers or larger, ISO 1 allows at most 10 per cubic meter and ISO 5 at most 100,000. Each step up, from ISO 1 to ISO 9, allows ten times as many. Big particles are always fewer than small ones, so within one class the limit for 0.5 micrometers and up is lower. In ISO 1 and ISO 2, which are cleaner than ISO 3, the limit for particles that big drops to single digits or isn't set at all, so those classes mostly look at smaller ones.

One more thing: the same cleanroom gives very different counts depending on whether people are working in it, because people are a big source of particles themselves (see Why do chip fab workers wear full bunny suits?). So a class comes with the state the room was in when it was measured: newly built with no tools yet, tools running but nobody inside, or a normal working day.

How does that compare with ordinary air?

Counting particles of 0.5 micrometers or larger, per cubic meter:

• Outdoor air in a city: often tens of millions, about the level of ISO 9.
• An ordinary office: one university cleanroom guide puts it at roughly 17 to 35 million.
• ISO 5 (Class 100): at most 3,520.

So ISO 5 is about ten thousand times cleaner than the air outside. If a cubic meter of outdoor air held ten thousand specks of dust, the same cubic meter in an ISO 5 room would hold one.

Why does a chip fab need to be this clean?

Because the lines on a chip are packed so tightly. On the most advanced chips, the densest lines sit only a few tens of nanometers apart (see The news says "2 nm". What exactly is 2 nm?). A 0.5-micrometer particle is 500 nanometers, more than ten times that spacing.

A particle like that landing on a wafer is like a boulder dropped on a circuit halfway through printing: it can break a line, or join two lines that should stay apart. That chip is dead, and it counts against yield (see What does "yield" mean in chip news?). So fabs care about more than 0.5 micrometers; smaller particles matter too.

How does a cleanroom stay this clean?

1. Most of the ceiling is filters: air goes through HEPA filters (which stop at least 99.97% of particles at 0.3 micrometers, the hardest size to catch) or even finer ULPA filters before it enters the room.
2. Air flows straight down: the filtered air moves down like a steady, even breeze. Particles from people and tools are carried down to the floor instead of drifting around, and are less likely to float sideways onto wafers.
3. The floor has holes: cleanroom floors are often raised and made of tiles full of holes. Air goes through the holes into the space underneath, travels back up through channels along the walls to the ceiling, and is filtered and blown down again. Most of the air just keeps circling like this; the cleanest cleanrooms replace all the air in the room a hundred times or more an hour.
4. The pressure is a little higher than outside: only by about one ten-thousandth of normal air pressure, but that's enough. When a door opens, air flows out, so dust from outside has a hard time getting in. The cleaner the area, the higher its pressure, so air always flows from cleaner places to less clean ones.
5. People and things go through checks: people change into a cleanroom suit and pass through an air shower that blows particles off the suit (how to put it on, and why, is in that article). Things do too: they are wiped clean first and passed in through a pass-through window. Ordinary paper, pencils and anything else that sheds stays outside.

Does the whole building need the top class?

Not necessarily. Making an entire large fab floor the cleanest class takes a lot of air and a lot of electricity. A common approach today is to save the strictest cleanliness for the few small spaces where wafers are actually exposed.

When wafers move around the fab, they ride in sealed pods (FOUPs), carried to the tools by an overhead hoist transport on rails in the ceiling. A pod opens only when it sits on the tool's load port (the platform at the tool's door), and a robot arm takes the wafers straight into the tool without them meeting the room's air. The pod itself is sealed, and the short stretch at the tool's door, where the arm moves the wafers, has its own filters blowing clean air. Those small spaces can be cleaner than the room. So the aisles where people walk don't have to reach the top class. They're still a cleanroom, though, and the suit still goes on.

In fab24hr

In fab24hr, every role changes into a cleanroom suit and goes through the air shower before walking into the cleanroom; tap the air shower and its card explains that "high-speed clean air blows the particles off you." As the process engineer, once ETCH-03 is repaired on the morning shift, the qual run is yours: "Particles added" must be 10 or fewer to pass. That counts how many particles landed on one wafer, which is a different count from the cleanroom's air class. The morning and afternoon shifts can also bring a monitor wafer with too many particles, and you decide whether the tool stops; on the Thin Film equipment engineer's afternoon shift, the test wafers come out with more particles than usual after a target change. Open "Know a bit more" on the Particle card and the ISO classes are there too. The fab, people, tools and numbers in the game are fictional, and the game doesn't say which class its cleanroom is; how the classes work follows the definitions published in the international standard. For other jargon, see Chip fab jargon in plain English.