Why do semiconductor fabs use so much electricity?

A chip fab uses so much electricity because two big blocks of it run all the time. One is the process tools that make the chips: they turn gas into plasma, heat wafers and pump chambers down to a vacuum. The other is the Facilities systems that keep the whole fab alive: cleanroom air handling, the chillers that make chilled water, ultrapure water, cooling water and exhaust treatment. Several public studies, from different years and with different methods, land in the same range: process tools take roughly 40 percent to half of a fab's electricity, and most of the rest goes to Facilities, with air handling and chilled water often the largest part. And none of it is switched off at any time of year.

Where does a fab's electricity go?

What uses power	What it does
Process tools	Plasma, heating, vacuum pumps, the light source for exposure
Cleanroom air handling	Blows filtered air downward nonstop and holds temperature and humidity steady
Chillers	Make chilled water to cool the air handling and the tools
Other Facilities systems	Ultrapure water, cooling water, exhaust, exhaust treatment, compressed air

For what these systems do, see "What does a fab facilities engineer do?"; for ultrapure water, "How pure is ultrapure water?"; for what else is in a fab, "What is a fab in semiconductors?". Here we only look at why they use so much power.

Why does cleanroom air handling use so much power?

The air conditioner at home only has to keep the room cool. A cleanroom's air handling has to do four things at once:

1. Keep the air moving: filtered air blows down from the ceiling, goes out through the perforated floor and is filtered again. In the cleanest areas, all the air in the room is replaced a hundred times or more an hour (see "How clean is a cleanroom?"). The fans run 24 hours a day.
2. Hold temperature and humidity steady: some steps, such as lithography, are sensitive to both. Outside it gets hot and cold, dry and humid, but inside it has to stay the same.
3. Keep bringing in fresh air: the tools' exhaust pulls air out all the time, and the room must also stay at a slightly higher pressure than outside. The missing air comes in from outside, and every bit of it has to be filtered, then cooled and dried, or heated and humidified, depending on the weather.
4. Carry the tools' heat away: almost all the electricity a tool uses ends up as heat. The air handling and the cooling water have to carry it out, and the chillers that make the chilled water use electricity too. So for every unit of power a tool uses, the fab spends extra power moving that heat out.

A US national laboratory found that buildings with cleanrooms and laboratories can use from several times to a hundred times the energy of an ordinary commercial building, for the same floor area.

Where does the tools' power go?

• Plasma: many etch and film deposition steps use high-frequency power to turn gas into a plasma (see "What is etching?"). One tool often has several chambers.
• Heat: a furnace heats a large batch of wafers to a very high temperature (see "What does a furnace do in a chip fab?").
• Vacuum pumps: many steps happen in a vacuum. The pumps mostly sit downstairs in the subfab and run continuously, and the exhaust treatment next to them, which burns or breaks down the waste gas, takes energy too.
• Exposure: the light used by the most advanced scanners is very hard to make, and for all that effort, only a small share becomes usable light (see "What is extreme ultraviolet (EUV) light?").

Why do more advanced processes use more power?

Each new generation of chip uses less power per switch (see The news says "2 nm"). But making it takes more:

• More steps: with finer lines and more layers, one layer often has to be printed several times and etched several times, with more cleaning and more measuring around it. Every extra step is more tool time, and the air handling and pumps run longer along with it.
• Some tools use more power: the most advanced scanners, for example. But they can merge several exposures into one, so the steps around them shrink too, and so does the power those steps would use.

Analyses by a research institute and by university researchers both found that for the same area of chip, each new generation takes more electricity to make.

Why not switch things off to save power?

The big reasons are in "Why do chip fabs run 24 hours a day?": the tools are expensive, wafers take months to go through, and a stopped fab is hard to restart. Power adds two more:

• Turn off the air handling and the cleanroom stops being a cleanroom: particles, temperature, humidity and pressure difference all drift. It has to be restarted and checked again before wafers can go back in.
• Idle tools still use power: the vacuum has to be held, and furnaces are often kept warm. The exhaust for toxic gases can't stop even for a moment (see "What gases does a chip fab use?").

Why is a power cut or a voltage dip so serious?

When the outside supply voltage drops suddenly, usually for less than a second, it's called a voltage dip. At home the lights flicker and that's it. In a fab, it can stop a tool: if just one small part inside, such as a control circuit, can't ride it out, the whole tool stops.

A stop costs three things:

1. The wafers inside: the ones stopped mid-step have to be measured before anyone knows whether they can be used, and some can only be scrapped.
2. The restart: the tool has to be restarted and pass a qual run before it can be released. A mild case takes hours, a bad one longer.
3. Downstairs has to come back too: pumps, exhaust, exhaust treatment and cooling water can trip as well. Until they are back, a tool can't run even if it's on, and it may trip again.

That's why the industry wrote a standard asking tools to ride out some short dips, and important equipment is often backed by uninterruptible power supplies or generators. In a real power cut, safety comes first: the toxic and flammable gas supply closes by itself, emergency power keeps the lighting, fire protection, toxic gas exhaust and gas detection running, and gas comes back only after checks.

How do fabs save power?

• Reuse heat: the heat carried out of the fab doesn't have to be thrown away. Outside air often has to be reheated after it's dried, and using recovered heat for that means less boiler or electric heating. Air on its way out can also trade heat with the air coming in.
• Move just enough air: a fan's power doesn't follow airflow one for one. Cut the airflow by a tenth and the fan's power can drop by 20 to nearly 30 percent. Fans and pumps also often have variable-speed drives, so they run as fast as needed, not flat out all the time.
• Shrink the cleanest space: wafers travel in sealed FOUPs that open only at the tool, so the strictest cleanliness is kept for that small space (see "How do wafers move around a fab?"). The room as a whole doesn't need such strong airflow, which saves a lot of power.
• Chilled water at two temperatures: not every place needs the coldest water. Using very cold water only where it's needed and less cold water elsewhere makes the chillers' job easier.
• Idle tools rest together: an industry standard defines an energy-saving idle state for tools. When a tool isn't processing, it can also tell the pumps and exhaust treatment downstairs to slow down, and bring everything back to normal when wafers are on the way.

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. On the late-night shift, "Night watch", you read a wall in the control room, with one tile each for power, cooling water and cleanroom temp & humidity. Tap "Power" and it says the power coming in from outside is steady and hasn't dipped once. On the morning, afternoon or evening shift you may get the random event "Voltage dip": the lights flicker for less than a second, and the control room says three tools and a pump tripped. You decide whether to confirm exhaust, exhaust treatment, cooling water and pumps are all back before letting the areas release their tools, or to let the areas release them first. As the etch process engineer, you may meet the same event from the tool side. The fab, people and tool numbers in the game are fictional, and the game doesn't say how much power this fab uses; where a fab's electricity goes and why it can't stop come from public, general sources. For other jargon, see Chip fab jargon in plain English.