How pure is ultrapure water, and why do chip fabs use so much of it?

Ultrapure water is water with almost everything except water molecules taken out: dissolved ions, particles, organic matter, oxygen and bacteria are all brought down to extremely low levels. Chip fabs use it to rinse wafers. How pure it is usually gets read as "resistivity". Perfectly pure water at 25 °C has a resistivity of about 18.2 MΩ·cm (megohm-centimeters). That is the theoretical limit, and fabs want water right up against that number. Tap water has a thousandth to a ten-thousandth of it. Wafers are rinsed many times in a fab, so an advanced fab uses several million gallons of ultrapure water a day, and making ultrapure water uses up even more raw water.

Why do wafers need such pure water?

On its way through the fab, a wafer soaks in chemicals again and again: to wash off particles, organic matter and metals, or to dissolve a layer of material (wet etching, see What is etching? How are dry and wet etching different?). After each bath, the chemicals have to be rinsed off right away. The last thing to touch the wafer surface is that rinse water, and whatever is in the water can stay on the wafer.

The finest lines on a chip are a few tens of nanometers wide or less, so a tiny amount of anything causes trouble:

• Ions: ions such as sodium that get into a transistor's very thin insulating layer shift how the switch behaves; metals such as iron and nickel that get into the silicon make transistors leak.
• Particles: one landing on a wafer can break a line or join two lines together. How clean is a cleanroom? covers why, and particles in water are the same problem.
• Organic matter: it leaves an invisible thin film on the surface, and the next layer of material doesn't grow well on it.
• Dissolved oxygen: bare silicon, just cleaned of its oxide, slowly grows a very thin oxide again when it sits in water that has oxygen in it.
• Bacteria: alive or dead, they are particles and organic matter, and they grow wherever water stands still.

So ultrapure water isn't just "water without salt". All five have to be controlled.

Why is purity read as resistivity?

Pure water molecules barely conduct electricity at all. Tap water conducts because it has calcium, magnesium, sodium and chloride dissolved in it as tiny charged particles called ions, and the current rides on them. So the fewer ions in the water, the harder it is for electricity to get through, and the higher the resistance. Resistivity turns "how hard it is for electricity to pass through a standard-sized piece of water" into one number.

Even with every other ion removed, water doesn't become a perfect insulator: a tiny fraction of the water molecules split by themselves into hydrogen ions and hydroxide ions. Those few ions set the ceiling, which at 25 °C is about 18.2 MΩ·cm. Tap water is only around a few thousand to twenty thousand ohm-centimeters, somewhere between a thousand and ten thousand times lower.

Resistivity is very sensitive. A trace of ions and it drops; even carbon dioxide from the air dissolving into the water makes it conduct a little more. That's why fabs mount resistivity meters right on the pipes and measure all the time. When resistivity falls, ions are getting in, for example because the ion-removing stage is nearly used up.

Resistivity only sees ions, though. Particles, organic matter, dissolved oxygen and bacteria each need a different instrument.

How is ultrapure water made?

Roughly, one stage after another, each taking out something unwanted. The details differ from fab to fab, but these are the common stages:

1. Filter first: the raw water has sand and suspended matter filtered out, so it doesn't clog the membranes later on.
2. Reverse osmosis: pressure pushes the water through a very fine membrane. Most of the dissolved salts and organic matter can't get through and stay on the other side.
3. Ion exchange: the water flows through tanks full of tiny resin beads. The resin grabs the remaining ions and releases hydrogen ions and hydroxide ions in their place, which join back up into water. When the resin is full, it is replaced or regenerated.
4. Ultraviolet light: strong UV light breaks up the last traces of organic matter and kills bacteria at the same time.
5. Degassing: the oxygen and other gases dissolved in the water are drawn out.
6. Final filtering: a membrane with even finer pores stops the smallest particles.

The finished water doesn't sit in a tank waiting. It circulates around a loop of pipe: out to every tool, with whatever isn't used flowing back to be treated again. Water that keeps moving gives bacteria less chance to grow. Even the pipe material is chosen with care, because the pipe wall itself can release a little into the water, so specially chosen plastic pipe is used rather than ordinary metal pipe. Like the rest of the fab, this system never stops (see Why do chip fabs run 24 hours a day?).

Why do chip fabs use so much water?

Three main reasons:

1. Lots of rinsing: a chip is built up one layer at a time, and every layer means several cleans; cleaning is one of the most repeated steps in the whole flow (see What do the seven process areas of a chip fab do?). After every chemical bath comes an ultrapure water rinse, and there are many thousands of wafers moving down the line at any moment.
2. Making ultrapure water uses more water: the salts and impurities that reverse osmosis holds back leave in a more concentrated stream of water that is drained away. So to make one unit of ultrapure water, a fab has to take in more than one unit of raw water.
3. Not just for rinsing wafers: the tools' heat is carried away by cooling water and finally released in cooling towers, which evaporate a lot of water; some tools' exhaust treatment also uses water to wash the exhaust. That water doesn't need to be so pure, but there is a lot of it.

Ultrapure water alone comes to several million gallons a day at an advanced fab. A gallon is about 3.8 liters, and an Olympic-size swimming pool holds about 2.5 million liters (about 660,000 gallons). So every day's ultrapure water would fill several swimming pools.

Where does used water go? Can it be recycled?

Yes, and fabs care about it a lot. Used water isn't all poured together; it is collected separately depending on how dirty it is:

• Cleaner water: the last few rinses, for example, which pick up only a tiny amount. After checking and treatment, it can go back as feed water for making ultrapure water.
• Less clean water: used once more where purity matters less, such as cooling towers and exhaust scrubbing.
• Water carrying more chemicals: treated until it meets discharge rules before it leaves the fab.

One of the hardest things for recycling is small organic molecules that carry no charge, such as some solvents used in cleaning. Reverse osmosis and ion exchange both struggle to catch them, so once they get into the reclaimed water, they can travel all the way into the ultrapure water. That's why reclaimed water is measured all the time, and when something looks wrong, it isn't taken back.

In fab24hr

In fab24hr, ultrapure water is something Facilities looks after. 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. On the late-night shift you're on night watch in the control room: one whole wall shows the fab's water, power, gases and chemicals, and one tile is the ultrapure water, its resistivity sitting at the top of the scale. Around four in the morning, the pure water system makes a routine switchover to its other set of equipment, and afterwards you check whether resistivity moved. As the etch process engineer or in Manufacturing, the morning and afternoon shifts can bring the random event "Ultrapure water resistivity dropping", and you decide whether to stop wet clean and wait for Facilities, or keep cleaning. The wet clean process and equipment engineers look after WET-05: a row of tanks where wafers soak in chemicals, rinse in ultrapure water and spin dry. The fab, people, tool numbers and times in the game are fictional, and the game doesn't say how much water this fab uses or recycles; how pure ultrapure water is, how it is made and why fabs use so much come from public, general sources. For other jargon, see Chip fab jargon in plain English.