Where does the air in a cleanroom come from?

Most of the air in a cleanroom is the same air going round and round. A ceiling full of fan filter units (FFUs) filters it and blows it down, it goes through holes in the floor, and it comes back up to the ceiling to be filtered and blown down again. Only a small share comes from outside: the tools exhaust air all the time, and a makeup air unit (MAU) brings in the same amount from outside, filtered, dried and set to the right temperature, plus a little extra so the cleanroom stays at slightly higher pressure than outside. The chilled water that cools and dries the air is made by chillers, and the heat finally leaves through cooling towers on the roof. All of this is cleanroom HVAC (heating, ventilation and air conditioning), and it belongs to Facilities.

How does the air move? Follow it step by step

An air conditioner at home only has to keep you cool. Cleanroom HVAC has to keep the air clean and hold temperature, humidity and pressure steady, all year round, without stopping.

Step 1: Outside air goes into the makeup air unit first

• What happens: Outside air is pulled in, passed through several filters, then cooled with chilled water to below its dew point (cool air to its dew point and the water vapor in it condenses into droplets). The droplets drain away and the air is dry. If it's now too cold it's warmed back up; if it's too dry, a little moisture is added.
• Why: Outside air carries dust, and it swings between hot and cold, dry and humid. Most of the moisture in a cleanroom comes in with it, so the hardest part, drying the air, is done here first.
• If you skip it: Damp air goes straight in, and the cleanroom's humidity follows the weather. Rain and fog show it most.

Step 2: It joins the cleanroom's big loop

• What happens: The treated air goes into the big loop that keeps circling through the cleanroom, and mixes with the air coming back up from under the floor. The loop moves far more air than the makeup air unit adds.
• Why: Clean air comes from filtering the same air over and over, not from swapping in new air. Outside air has to be dried and cooled, which is the hardest work, so only as much as needed is brought in.
• If you skip it: With nothing but outside air, the drying and cooling would be more than anyone could afford. HVAC is already one of the big electricity users in a fab (see "Why do semiconductor fabs use so much electricity?").

Step 3: Fan filter units (FFUs) in the ceiling blow it down

• What happens: A fan filter unit is a small fan plus a very fine filter, fitted into one square of the ceiling grid. A cleanroom ceiling holds hundreds or thousands of them. The air is pushed through the filters and moves down like a slow, even breeze (a few tenths of a meter per second, slower than walking), and goes back through the holes in the floor.
• Why: People and tools shed particles all the time. The downward air carries them to the floor instead of letting them drift onto wafers. With one unit per square, if one fails, the ones around it keep blowing.
• If you skip it: When a unit stops, the air under that square gets weaker. And if a newly fitted unit leaves a gap between its frame and the ceiling, unfiltered air leaks down through the gap. That's why a new unit gets a "leak scan": a probe is moved slowly along the filter and the frame.

For the filters and how cleanroom classes are counted, see "How clean is a cleanroom? What Class 100 and ISO 5 mean".

Step 4: The pressure stays a little higher than outside (positive pressure)

• What happens: A little more air is brought in than is exhausted, so the cleanroom's pressure is higher than outside. This is called positive pressure. It's only about one ten-thousandth of normal air pressure higher, and the cleaner the area, the higher it is.
• Why: When a door opens, air flows out, and dust from outside has a hard time following it in. Many entrances have two doors that never open at the same time (the air shower too), so the pressure difference doesn't vanish all at once.
• If you skip it: Without the pressure difference, every open door and every gap in the walls lets outside dust ride in on the air. It works the other way for gas cabinets that must not leak toxic gas: they are kept at lower pressure than their surroundings on purpose (see "What gases does a chip fab use, and why so much care?").

Step 5: Temperature and humidity stay steady

• What happens: Sensors measure all the time and send the readings to the control room. The big loop commonly uses dry coils, kept just warm enough that no water condenses on them, to fine-tune the temperature; adding and removing moisture is left to the makeup air unit.
• Why: The litho area cares most. When the humidity changes, how well the photoresist sticks and, for some resists, how fast it reacts to light change with it. Exposure also has to line up each layer with the one below to within nanometers, and things expand a tiny bit when they warm up, so the temperature has to hold still too. Air that's too dry builds up static; air that's too humid corrodes metal.
• If you skip it: The same settings give a different result today than tomorrow.

For litho and photoresist, see "Why is the lithography area in a chip fab yellow?" and "What is photoresist?"; for static, see "What is electrostatic discharge (ESD)?".

Step 6: The tools exhaust air all the time, and every bit is replaced

• What happens: Every tool is connected to exhaust ducts that keep pulling air out of it. The exhaust is split by what's in it; a common split is general exhaust, acid, alkaline and organic solvent. Toxic or flammable gases first go through exhaust treatment right next to the tool. Acid and alkaline exhaust is then washed with water (in a scrubber), and solvent exhaust is often captured, concentrated and burned before anything goes outside.
• Why: Chemical vapors can't stay inside the tool or escape into the cleanroom. The streams are kept apart because acid and alkaline gases react when they meet and form a fine salt powder that builds up in the ducts, and each one is treated differently.
• If you skip it: Too little exhaust and the vapors stay in the tool. Too much and the other tools on the same exhaust main get less, and the cleanroom loses air that the makeup air unit then has to replace. The scrubbers' water has to be treated too (see "Where does a chip fab's used water go?").

Step 7: Chillers and cooling towers carry the heat away

• What happens: The coils in the makeup air unit and in the big loop all run on chilled water from the chillers. The chilled water picks up heat from the air and goes back to the chiller, which hands that heat to a second water loop and sends it to the cooling towers on the roof. There, warm water is sprayed down from the top, fans pull air upward, and a small part of the water evaporates and takes the heat with it.
• Why: Nearly all the electricity the tools use ends up as heat, and it has to be moved out all the time.
• If you skip it: If a cooling tower fan stops, the water going back isn't cool enough, the pressure inside the chiller rises, and the chiller shuts itself down to protect itself. And when the chilled water isn't cold enough, the makeup air unit can't dry the air. That's why there's usually a spare chiller standing by.

In fab24hr

Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to the Facilities engineer. In the row of color tiles under the name, tap the HVAC one (it reads "Facilities · HVAC"), and you're the HVAC engineer. Which of the four shifts you get follows the time you open the game:

• Late-night shift, "A Foggy Night": You're on-call at home. It's foggy outside and the humidity in the litho area is creeping up: the night setting raised the chilled water temperature a little, and the makeup air unit can't dry the air. You decide whether to ask the control room to go back to the daytime temperature or stay on the night setting and watch. At 04:30 there's another call (you can let it ring): a fan filter unit in the ceiling has stopped.
• Morning shift, "Swap One": You swap that unit in the cleanroom. The leak scan can be skipped, and if you carry on into the afternoon shift you'll see what that does. Then you look at seven nights of humidity and decide whether the night setting should follow the outside dew point.
• Afternoon shift, "The New Tool’s Exhaust": A new tool comes in through the move-in entrance, and you watch the pressure difference as the two doors open. Then come its two exhaust lines. The vendor wants them opened wider; whether you follow the connection list and whether you measure the airflow is up to you.
• Evening shift, "One Fan on the Roof": Chiller 2 trips and the standby, chiller 3, starts on its own. The cause is a fan on one of the roof's cooling towers, and checking the belt is another step you can skip.

The fab, people, tool numbers and times in the game are fictional, and it gives no numbers for temperature, humidity or pressure difference. How the air moves and why it's done this way come from public, general sources and don't describe any particular company. For other jargon, see Chip fab jargon in plain English.