Where does a chip fab's used water go?

A chip fab doesn't pour all its used water down one drain. It is sorted right at the tool by what has been added to it. The cleaner rinse water is reclaimed, treated and sent back to make ultrapure water. Water carrying acids and alkalis, fluoride, polishing particles, metals or organics runs in its own pipes to the wastewater plant, gets the treatment that fits it, and is only discharged, meaning let out of the fab, once tests show it passes. That is what people mean by semiconductor wastewater treatment and fab water recycling. Separately, the tools have their own cooling water, which never touches a wafer: it circles in closed pipes and carries the tools' heat away.

What's in a fab's used water?

Every time a wafer comes out of a chemical bath, it is rinsed with ultrapure water (how that water is made: How pure is ultrapure water?). The rinse water carries off whatever was on the wafer, so "used water" can mean very different things:

• The last few rinses: only a trace more than before; almost clean.
• The first rinses after a chemical bath: they carry acid or alkali (see What is wafer cleaning?).
• Steps that use hydrofluoric acid: the water contains fluoride. Hydrofluoric acid dissolves silicon dioxide; see What is etching?.
• Rinse water after CMP: full of very fine polishing particles (see What is CMP?).
• Developing and solvent steps: the water contains organic compounds.
• Steps that build metal wiring: the water may carry metals such as copper.

It's a bit like sorting trash at home: recycling, food scraps and general trash each go somewhere different, and if it all goes in one bag, the recyclables are ruined too.

Where does the used water go, step by step?

Step 1: Sort it right at the tool

Each tool's drains are piped by type into different lines. That way every kind can get the right treatment, and the cleaner stream stays clean. Mix them, and the clean water gets dirty and can't be reclaimed, there is more water to treat, and some things become harder to remove once they are diluted, fluoride for example.

Step 2: Keep measuring the reclaim line's conductivity

The reclaim line has an instrument that measures the water's conductivity all the time: the more ions (tiny charged particles) are dissolved in the water, the better it conducts electricity (the resistivity used for ultrapure water is the same thing turned upside down). If the reading climbs and the water is still taken back, the dirt travels into the pure water system and the reverse osmosis at the front has to work harder and harder. So once it goes out of range, the line is switched to the wastewater plant, and reclaiming starts again when it is clean.

Step 3: Send reclaimed water to the front of the pure water system

The reclaimed water is filtered and treated, then joins the raw water drawn in from outside at the front of the pure water system and goes through it stage by stage to become ultrapure water. Most of the time it is cleaner than ordinary raw water, so it is easier to treat, and the fab draws a little less water from outside. It can't be used to rinse wafers directly: reclaimed water is only "cleaner", not ultrapure.

Step 4: Everything else first goes into a big mixing tank

Wastewater from all over the fab first flows into a big tank, the equalization tank, where it is mixed before being fed on to treatment a little at a time. Wastewater arrives in surges and lulls, and its strength rises and falls, so evening it out lets the chemical dosing downstream keep up. If too much arrives at once, for example when a wet clean tool changes its bath and drains the old chemicals in one go, the tank fills, the chemicals can't react in time, and the water at the outlet may fail its tests. So before a big discharge, the wastewater plant is told in advance, and the drain is done in batches.

Step 5: Treat each kind its own way

No single method handles everything, which is why the water was sorted in the first place. Every fab does it a little differently; the common principles are:

• Acids and alkalis: add alkali or acid to neutralize it to near neutral.
• Fluoride: add calcium. Calcium and fluoride combine into calcium fluoride, a solid that doesn't dissolve in water, so it settles and can be separated out.
• Polishing particles: they are tiny and carry an electric charge, so they push each other away and stay floating. Chemicals are added to make them stick together into big clumps, and only then do they sink.
• Metals such as copper: turn them into a solid that settles out, or catch them with resin.
• Organics: much of it is left to microbes to eat slowly; some of what they can't break down easily is first broken apart by oxidation.

What settles out becomes a sludge, which is pressed to squeeze out the water and then taken away for disposal.

Step 6: Test it, and let it out only if it passes

Before treated water is let out, it is tested, for example for acidity and how much fluoride is left, and it is released only if it is within range. If it isn't, the outlet is shut and the water is sent back to be treated again. Where it goes and how clean it has to be depend on local rules.

Can all of a fab's used water be recycled?

No. Cooling towers evaporate some water; what reverse osmosis holds back is carried off in a more concentrated stream; and some things are so hard to remove that reclaimed water containing them can't be taken back (see How pure is ultrapure water?). How much a fab reclaims varies a lot from fab to fab, and the numbers are counted in different ways, so this article gives none.

Is the tools' cooling water the same as ultrapure water?

No. Cooling water never touches a wafer; it is a separate, closed loop. It leaves the cooling water plant, splits off the main pipe into branch after branch, goes into each tool, flows past pumps, power supplies and chambers to carry off their heat, and comes back to the cooling water plant. There it hands its heat to colder chilled water through a heat exchanger (the two streams never mix; only the heat crosses), cools down, and goes out again. Chilled water is made by chillers, and the air conditioning uses it too (see Why do semiconductor fabs use so much electricity? and Where does the air in a cleanroom come from?).

Cooling water is treated too. It doesn't have to be as pure as ultrapure water, but if it gets dirty it leaves scale and grows things, so it carries heat away less well, and the narrow water channels inside a tool can clog.

On the supply side, Facilities watches the pressure, the temperature and the flow in each branch. Many tools also watch their own cooling water flow: if it drops far enough, the tool raises an alarm and stops before it overheats and is damaged.

When the main pipe is fine and only one branch runs short, the problem is usually in that branch: a valve not fully open, a clogged strainer, a fitting with a problem. Turning up the supply pressure for the whole loop may not get more water to it, and the other tools on the same loop get extra water first. It's like one apartment in a building with a weak tap: you check that apartment's pipes, not raise the water pressure for the whole building.

In fab24hr

Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to the Facilities engineer and tap "Facilities · Water" under the name: you're the Water engineer (marked Beta for now), and the ultrapure water, the cooling water and the wastewater all belong to your team. Which of the four shifts you get follows the time you open the game:

• Late-night shift, "Two Trains, Taking Turns": you're on call at home. At four in the morning the pure water system makes its routine switchover to Train B. Resistivity doesn't move, but TOC (total organic carbon, how much organic matter is in the water) creeps up: Train B's UV lamps, which break down organics, have been in use a long time. You can ask the control room to switch back to Train A, or keep Train B on and change the lamps in the morning. At five the control room calls (you can let it ring): CVD-02's cooling water branch is running short, and Equipment asks whether the supply pressure for the whole loop can be raised.
• Morning shift, "The Acid Change": wet clean tool WET-05 is changing its bath, and the old bath has to drain to the wastewater plant, where the equalization tank is already running high. You decide whether it drains all at once or in two batches.
• Afternoon shift, "Water for a New Pipe": a new tool's ultrapure water connection is in. Next to it is a short pipe stub left for another tool and never used; water sits in it without flowing, which is called a dead leg, and standing water grows bacteria and collects particles. You decide whether to cut it off now or leave it capped, and the vendor also wants to hook the tool up and try the water early, which you can turn down.
• Evening shift, "The Reclaim Line": WET-05's new bath runs its first lot of the evening, and conductivity rises on the rinse water reclaim line. The control room asks over the radio (you can leave it unanswered): send the line to waste for now, or keep reclaiming and watch it.

The fab, people, tool IDs and times in the game are fictional, and the game doesn't say how much water this fab uses, how much it reclaims or what the discharge limits are. How water is sorted, treated and reclaimed comes from public, general sources and doesn't represent any one company's practice. For other jargon, see Chip fab jargon in plain English.