What is electrostatic discharge (ESD), and why are chips so sensitive to static?

Electrostatic discharge, ESD for short, is charge that has built up on a person or an object jumping all at once to something else it touches or comes very close to. The snap you get from a door handle in winter is one electrostatic discharge. To you it's just a sting. To a chip it can be like a lightning strike: the finest parts of a chip are more than a thousand times thinner than a hair, and a sudden rush of charge can punch through them or burn them out.

Where does static come from?

When two materials touch and then separate, some electrons move from one to the other: the side that gains electrons ends up negative, the side that loses them ends up positive. Rubbing moves even more electrons, which is why it's often called charging by friction. Walking across a carpet, standing up from a chair, pulling off a strip of tape or taking off a sweater all do it.

If the thing doesn't conduct electricity, like plastic, synthetic clothing, or a person in rubber-soled shoes, the charge stays put with nowhere to go. That's why it's called "static". The drier the air, the longer the charge stays: on a dry day, a person walking across a carpet can build up more than ten thousand volts.

And what is a discharge?

When a charged thing touches something that conducts, such as a metal door handle, the charge rushes across in one go. That's the discharge. The voltage is high, but it's over far faster than a blink and not much charge moves, so a person only feels a sting.

The trouble is that people only notice it at around two or three thousand volts, while some very sensitive chips can be harmed by a few tens of volts. A discharge you never felt may already have damaged a chip.

Why are chips afraid of static?

The basic part inside a chip is the transistor, a tiny switch controlled by electricity (see "What is a transistor?"). Under its gate sits a very thin insulating layer, the gate oxide, which normally only has to hold back the small voltage the chip runs on. A discharge suddenly pushes in a very high voltage, and the usual kinds of damage are three:

• The insulating layer is punched through: like a hole knocked in a thin wall, the gate connects to what's underneath, and the switch stops doing what it's told.
• A spot in the silicon is burned: the current crowds into a tiny area and melts a little silicon, leaving a path for leakage or a short.
• A wire burns out: often after one of the first two has failed, a large current surges through a fine metal line and melts it.

The smaller chips are made, the finer the parts inside, and the less they can take.

Does a zapped chip always fail on the spot?

Not always. Some fail on the spot: they don't work at all, or one pin is shorted, and tests before shipping usually catch them (see "How are chips tested?"). Others are only wounded: they still work and pass the tests, but a small injury is left in the insulating layer, with a little more leakage, and they fail after a while. You can't see it, which makes it the hardest kind. Whether a chip can withstand discharges is also one of the things reliability testing checks (see "What is reliability testing?").

Does a chip protect itself?

Yes. Every pin has a protection circuit built next to it. Normally it does nothing; when a very high voltage suddenly arrives at the pin, it opens a path that sends the charge to the power line or the ground line, around the transistors inside, like a spillway beside a river.

But it can't be made as big as you like: make it bigger, and fast signals on the pin are slowed down. So it is the last line of defense, and static still has to be controlled outside the chip first.

How do factories prevent static, step by step?

Places where chips are handled, like the assembly line and test area at a test-and-assembly house, are set up as static-protected areas. The idea: don't let charge build up; if it does, let it drain away slowly; if it can't drain, neutralize it; and finally pack the chips before they leave.

Step 1: Ground the people: wrist straps, mats, floors and shoes

• What: a strap on the wrist with a cord to a ground point on the bench; the bench mat, the floor and the shoes are made to conduct slowly too.
• Why: people pick up charge whenever they move; grounded, the charge drains away as soon as it forms. The cord has a large resistor in it so the charge flows slowly, and so that a person who touches something live isn't hurt.
• If not: you reach for a chip, and the charge on your body jumps from your fingertip into its pins.

Step 2: Put on an anti-static smock

• What: over your own clothes, a coat made of fabric that doesn't hold charge and lets it drain slowly, fully buttoned.
• Why: sweaters and synthetic clothing charge up as you move, and a wrist strap can't reach the charge on your clothes; covered up, that charge isn't carried next to the chips.
• If not: your sleeve brushes the bench, and the charge on your clothes comes close to the chips anyway.

Step 3: Keep charging materials away, and make what touches chips conductive

• What: things that don't conduct, like plastic cups and tape, stay away from the chips; the tools, trays and machines that touch chips use slowly conducting materials and are grounded.
• Why: grounding can't pull charge off something that doesn't conduct; and as chips are picked up and put down inside machines, they get charged themselves.
• If not: a charged chip touches metal, the charge rushes out of the chip itself, and it's damaged just the same.

Step 4: Neutralize what can't drain away, with ionizers

• What: ionizers on the bench or inside machines blow out air molecules carrying positive and negative charge.
• Why: plastic and glass can't be grounded; charged air molecules are drawn to spots with the opposite charge and cancel it.
• If not: the charge just stays there. Ionizers need regular checks too: if positive and negative aren't balanced, they can end up charging things.

Step 5: Keep the air from getting too dry

• What: the air conditioning controls humidity so it doesn't drop too low.
• Why: in slightly moist air, charge dissipates more easily; the drier the air, the more charge the same movement creates.
• If not: in dry seasons, static causes far more trouble. But too damp is no good either: a chip's plastic package doesn't like absorbing moisture.

Step 6: Pack chips in anti-static trays and bags before they leave

• What: chips go into anti-static trays or tubes, then into an anti-static bag with a thin metal layer inside. The bag often carries a triangle with a crossed-out hand in it: sensitive to static, don't touch without protection.
• Why: the metal layer acts like a cage, so a discharge from outside travels around the surface and can't get in; the inner layer of the bag is also made not to charge by rubbing.
• If not: an ordinary plastic bag charges up when rubbed, which is like wrapping the chips in something that makes static. Chips that dislike moisture also go into a moisture barrier bag; see "How are chips shipped?".

Do wafer fabs need to guard against static too?

Yes, though what they fear most is a little different:

• Wafers: for a wafer still being made, the bigger worry than discharge damage is that a charged wafer pulls particles out of the air.
• Photomasks: the metal patterns on a photomask are separate islands that can't be grounded; once charged, a discharge can jump between two that sit very close and spoil the fine pattern (see "What does a mask engineer do in a chip fab?").
• Tools: the electrical noise from a discharge can make a tool lock up.

Hold on, don't etch tools use static on purpose to hold wafers?

They do. Inside an etch tool's chamber it's close to a vacuum, so a chuck that holds by air pressure won't work. It uses an electrostatic chuck (ESC) instead, which holds the wafer flat with static (see "What is plasma, and why do chip fabs etch with it?").

Static has two sides. Charge that stays put attracts things, the way a balloon rubbed on your hair can lift it. Charge that suddenly runs off is a discharge. The chuck uses the first side on purpose. What factories guard against is the second side: the uncontrolled moment when charge suddenly rushes through a chip.

In fab24hr

Tap "Clock in", choose "A day in the fab", tap "Test-and-assembly house" on the "Your role today" card and swipe to the test engineer or the assembly process engineer. The person on the card wears an anti-static smock down to the thighs and thin gloves, and the assembly engineer also has a hair cap. Inside the test-and-assembly house you wear the same smock in the office; when you walk into the cleanroom, you change into a cleanroom suit.

You can see an electrostatic chuck too. As the etch equipment engineer at the production fab, your morning shift opens a chamber and replaces the old chuck. And under "Learn" on the home screen, tap the etch chamber to take it apart and see the chuck under the wafer.

The fabs and people in the game are fictional, and there is no storyline about static; what electrostatic discharge is and how factories guard against it follow public, general sources. For other jargon, see Chip fab jargon in plain English.