What gases does a chip fab use, and why so much care?

A chip fab uses two broad kinds of gas. Bulk gases such as nitrogen, oxygen, argon and hydrogen are used in huge amounts and come in by pipe from large tanks or gas plants outside the building. Specialty gases, such as chlorine for etch and silane for growing thin films, are used in small amounts, but some are toxic, some are corrosive and some catch fire in air. Most are fed one cylinder at a time from gas cabinets. Both kinds have to be extremely pure, so the fab watches them with layer upon layer of equipment and procedures.

Why does a fab need so many gases?

Because many of its steps happen in gas. A furnace uses oxygen to grow an oxide layer. Thin film tools let gases react on the wafer surface to lay down a new layer of material. Etch turns gas into a plasma that carries material away as gas. Ion implant turns gases containing boron or phosphorus into ions and fires them into the silicon.

Inside a tool, a flow controller sets how much of each gas flows, usually measured in sccm.

What are bulk gases, and what are they for?

Bulk gases are like tap water at home: used in large amounts, all the time, there whenever you turn them on.

Gas	Common uses
Nitrogen	It hardly reacts with anything, so it pushes air and moisture out of lines and tools and protects things that would oxidize
Oxygen	Grows silicon dioxide on silicon in a furnace (see What does a furnace do in a chip fab?)
Argon	Doesn't react and its atoms are heavy enough, so in sputtering it is turned into a plasma that hits the target (see What is thin film deposition?)
Hydrogen	Some film steps use it to carry other gases into the chamber, and some heating steps run it too

Nitrogen, oxygen and argon all come from air. Air is roughly four-fifths nitrogen and one-fifth oxygen, with under 1% argon. Cool the air until it turns liquid, then separate it by boiling point, and you get all three. Helium is common too: it carries heat very well, so during etch it flows behind the wafer to cool it (helium backside pressure).

What are specialty gases, and why are they dangerous?

Specialty gases are process gases, and many of them are toxic, corrosive or flammable. Think of the locked cabinet at a pharmacy: there isn't much of each, every one has its own job, and every one has its own danger.

• For etch: gases containing chlorine are often used to etch silicon and metals; gases containing fluorine often etch silicon dioxide (for how plasma etches, see What is etching?). Chlorine is heavier than air, so a leak collects in low places, and it is strongly corrosive to moist tissue such as the eyes and throat.
• For thin films: silane supplies the silicon a film needs. In air it can ignite by itself, without any flame.
• For ion implant: gases containing boron, phosphorus or arsenic, and some of them are highly toxic. Government occupational safety guidance warns that one of them doesn't irritate and causes no immediate symptoms when breathed in, and that its smell is not enough to tell you it's there.

Bulk gases need care too. Hydrogen burns very easily: about 4% of it in air can be enough to ignite. Nitrogen isn't toxic, but a large leak in an enclosed space pushes the oxygen out. You can't smell it or feel it; only an instrument can tell.

So the rule is: don't rely on anyone's nose; rely on equipment and procedures.

How does gas get from downstairs to the tools?

A specialty gas roughly travels "gas cabinet → valve box → tool". Much of this sits on the subfab, the floor below the cleanroom, and is looked after by Facilities (for what else is in a fab, see What is a fab in semiconductors?).

1. Gas cabinet: the cylinders are shut inside a steel cabinet that is always being exhausted, so the air pressure inside is a little lower than outside. If something leaks, the gas is pulled away first. At the back, a panel of valves and regulators brings the cylinder's high pressure down. If the detector picks up a leak or the exhaust stops, the outlet valve closes by itself.
2. Valve box: where one gas line branches to several tools. Each branch has its own valve, and the box is connected to exhaust too.
3. Piping: lines for high-purity gas are mostly stainless steel tubing with a very smooth inner wall. Toxic gases often run in double-walled pipe: the gas flows in the inner pipe, the outer pipe catches a leak first, and a sensor watches the space between them. A fitting that can be taken apart has a metal gasket between its halves.

For how Facilities does its rounds and changes a cylinder, see What does a fab facilities engineer do?.

Why so much care? The protection comes in layers

Workplace safety often talks about an order: if equipment can keep people away from a hazard, use the equipment first; then rules and procedures; personal protective equipment is the last line. Gas protection is arranged the same way. What follows is the reasoning, not a set of instructions; every fab has its own procedures and training.

• Contain it first: exhausted gas cabinets with lower pressure inside, double-walled pipe, valves that close by themselves.
• Sniff all the time: a gas detector smells the gases people can't. It drifts over time, so a little test gas is puffed at it regularly to check that it still goes off (a bump test).
• Before a line is opened: lockout/tagout first, so the valve stays locked shut and everyone knows someone is working on it; then a purge clears the leftover gas out of the line with nitrogen.
• After it's connected: a leak check makes sure there is no gap at the fittings. A new line also gets a pressure test: it is filled with nitrogen and left closed to see whether the pressure slowly falls.
• The human layer: when a toxic gas cylinder is changed, a common practice is two people working together, wearing respiratory protection.
• What's used up still matters: not all the gas is used up in the tool. What's left, and what the reactions produce, first goes through exhaust treatment, where it is burned, broken down or washed with water before it leaves; some fluorine-containing gases are also powerful greenhouse gases.

Why do the gases have to be so pure?

The finest lines on a chip are only tens of nanometers wide, and some films are only a few nanometers thick. Whatever else is in a gas can follow it into the chamber and end up in a film or on the wafer surface. Moisture and oxygen are a particular worry: a tiny amount can make a film grow wrong, or oxidize something that shouldn't be oxidized.

Purity is often given as a number of nines: 99.9999% is "six nines", which leaves one part per million of impurities. The impurities a fab worries about are counted further down still, in parts per billion (ppb). How small is one part per billion? An Olympic-size swimming pool holds about 2.5 million liters of water; one billionth of that is about 2.5 milliliters, roughly half a teaspoon.

It isn't only the gas that has to be clean; the path does too. A pipe wall holds on to a little moisture and lets it out slowly, so the inside of the tubing is kept smooth and the layout avoids dead ends where gas doesn't flow. When a line is opened for a cylinder change, air and moisture can get in, so a purge protects not only the people but also every wafer that comes after. Water is the same story; see How pure is ultrapure water?.

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; you look after gases and chemicals. On the late-night shift, "Night watch", you read a wall in the control room, where bulk gases and specialty gases each have a tile. On the morning shift, "Cylinder change", you and Hui-Ju, both in respiratory protection, change the cylinder of specialty gas for etch in Cabinet B on the subfab; any step you skip comes back to find you later. On the afternoon shift, "Move-In", you hook up a new tool line by line, and you decide whether the new gas line's pressure test runs its full length. On the evening shift, "A Trace in the Cabinet", Cabinet B's detector picks up a trace, and you work out whether it's a real leak or the detector drifting. You can also take a gas cabinet apart in Chip fab machines, taken apart.

The fab, people, tool numbers and which gas is in which cabinet are fictional, and the game doesn't give figures such as how many purge cycles or what an alarm is set to; which gases a fab uses and why it is so careful come from public, general sources and do not describe any company's practice. For other jargon, see Chip fab jargon in plain English.