What are the rules for entering a cleanroom, and why no makeup or ordinary paper?

These are the usual cleanroom rules, sometimes called cleanroom etiquette: no makeup or perfume before you come in, and long hair tied back; change into a cleanroom suit and go through an air shower; inside, use only cleanroom paper and pens, wipe phones and tools first, move slowly, don't eat or drink, and don't touch wafers with your hands; on the way out, undress in reverse and leave the suit behind. All of them guard against one thing: keeping anything that sheds dust, flakes or fibers away from the wafers, and one of the biggest sources is people themselves. Every fab and lab has its own rules; what follows are the common practices in rules that universities and research labs publish, and the reasons behind them.

What do you do before going in? Why no makeup?

Step 1: Before leaving home, no makeup or perfume, and tie long hair back

• What: no foundation, face powder, eye shadow or mascara; no perfume or hair spray; long hair tied back.
• Why: a lot of makeup is very fine powder that comes off when you sweat or touch your face, and some of it is metal compounds, such as the iron oxides and titanium dioxide common in foundation and eye shadow. Perfume and hair spray give off tiny droplets, and a scent can also mask the smell of a chemical leak.
• If you don't: the suit leaves only a ring around your eyes uncovered, so powder from your face drifts out right there, and it also stains the suit.

Lotion is a special case: dry, cracked skin sheds flakes too, so some places actually allow approved lotions.

Step 2: Take off your shoes, cross the sticky mat, wash up, and put on the suit

• What: leave your shoes outside, step across a tacky floor mat (a sticky mat), and at some places wash your hands; coats, bags and books stay outside. Tuck all your hair, ears included, into a hair net, add a beard cover if you have a beard, then put on the suit from the top down, gloves last.
• Why: shoe soles carry dirt in from the street, and the sticky mat pulls some of it off; the head and face shed the most, so they get covered first.
• If you don't: the suit gets dirty before you've even finished putting it on, and a lock of hair sticking out of the hood keeps shedding particles.

How to put the suit on, and why in that order, is in "What is a bunny suit, and why do chip fab workers wear one?".

Step 3: Go through the air shower, stand in the middle and turn slowly

• What: you step into a small room, the door closes behind you, and nozzles on both sides blow strong, filtered air. You stand in the middle, arms raised, and turn slowly all the way around; only when it's done does the door on the other side open.
• Why: turning lets the air reach every side of you, blowing off some of the loose particles on the outside of the suit; and since the two doors never open at the same time (they're interlocked), outside air can't flow straight through.
• If you don't: only your front gets blown; lean against the wall, and the side touching it isn't blown at all.

How much an air shower actually removes has long been debated. It's an extra safeguard, not a substitute for wearing the suit properly.

What are the rules inside, and what can't you bring in?

Step 4: Write only on cleanroom paper, with a cleanroom pen

• What: write on special cleanroom paper with an approved pen, and wipe things with lint-free wipes; ordinary paper, notebooks, cardboard boxes, pencils, erasers and correction fluid stay outside.
• Why: ordinary paper is pressed wood fibers, and tearing, folding or turning a page sheds fibers and paper dust; writing with a pencil leaves a layer of graphite powder on the page, and an eraser leaves crumbs.
• If you don't: turning one page of notes scatters invisible paper dust next to the tool; documents that need to come in are often printed on cleanroom paper or laminated.

Step 5: Can you bring a phone? Wipe it first, or use a dedicated one

• What: phone rules vary a lot: some places let you bring one in after wiping it with alcohol at the entrance, some only allow phones outside, and some don't allow photos inside, for confidentiality. Tools and parts are wiped clean, taken out of their cardboard, and passed in through a pass-through (a small cabinet with a door on each side).
• Why: a phone spends all day in your pocket and hands and collects dust and grease, with even more in the gaps of its case, so some places ask you to take the case off.
• If you don't: you changed your clothes, but the dirt came in with your things.

Step 6: Walk slowly, never run, and don't lean on tools

• What: walk, don't run, move slowly, and don't lean or sit on tools or work surfaces.
• Why: the bigger your movements, the more particles get pumped out through the gaps in the suit, and running stirs up the air that flows from the ceiling down; leaning on something swaps dirt between your suit and the surface.
• If you don't: dust the airflow should carry to the floor gets kicked back up, floats a little longer, and has a little more chance to land on a wafer.

Step 7: Don't touch wafers with your hands, and change torn gloves

• What: wafers stay in sealed boxes (FOUPs) moved by machines whenever possible (see "How do wafers move around a fab?"); when one must be handled, you wear gloves and use a vacuum wand or special tweezers, touching only the back or the edge. If a glove tears or gets dirty, you go out and change it.
• Why: sweat and skin oil carry salt, and its sodium can work its way into the very thin insulating layer of a transistor and slowly shift how the switch behaves.
• If you don't: one invisible fingerprint can cause problems for the chips in that patch of the wafer.

Step 8: No food, no drink, no gum

• What: food, drinks and chewing gum stay outside.
• Why: crumbs and droplets are contamination, and there are chemicals in a cleanroom that could get on anything you put in your mouth.
• If you don't: the crumbs from one bite of a cookie are a whole handful of particles.

Step 9: Some areas have extra rules

• The litho area uses only yellow lights, to protect the photoresist on the wafers, which reacts to blue and ultraviolet light (see "Why is the lithography area in a chip fab yellow?").
• Where chips are handled, static is controlled, for example with grounded wrist straps (see "What is electrostatic discharge (ESD)?").
• Some areas need training and access rights first; people without access have to be escorted.

What do you watch for on the way out?

Step 10: Undress in reverse, and never wear the suit outside

• What: back in the gowning room, you take things off in roughly the reverse order (some places have you do it bottom-up with your gloves still on), throw away the mask and gloves, and hang the suit in its place or send it to be washed.
• Why: the outside of the suit has picked things up, and taking it off in order keeps that from touching the inside and the parts you'll wear again.
• If you don't: wear it outside and outside dirt gets on it, ready to ride back in next time; that's why some rules say plainly that the suit never leaves the cleanroom.

One exception: when a fire or gas alarm goes off, you don't undress in order, you just get out (see "Is working in a chip fab dangerous?").

What are all these rules keeping out?

The ten steps really come down to three things:

1. What people shed: skin flakes, hair, makeup powder, clothing fibers, all of them sources of particles.
2. What people bring in: paper, pens, phones, cardboard boxes, food. Swap in a cleanroom version where you can; wipe down what you can't.
3. What's already inside, kept from drifting and touching: move slowly, don't lean, don't touch the wafers.

The air itself is filtered by the air handling system (see "Where does the air in a cleanroom come from?" and "How clean is a cleanroom?"); the rules cover what that system can't: people and their things. One particle too small to see can kill a chip, and that's lost yield.

In fab24hr

Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to the Process engineer, and in the row of color tiles under the name tap Etch: you're the etch process engineer. Before you walk into the cleanroom you change into a cleanroom suit in one of five colors, such as "Rookie Yellow" for new hires and "Visitor Blue" for visitors and vendors. Tap the air shower, then "Take it apart" and "See it work", to watch someone turn slowly in the blast of air.

You may also find a small hole in the fingertip of your glove (go out for a new pair, or tape it for now?), pick up a cleanroom pen from the floor, or meet the vendor waiting outside the air shower: the part comes out of its outer box, gets wiped and goes in through the pass-through. Play the morning shift as the vendor field service engineer on the R&D line and you're the one who can't get in: your camera phone goes into a locker at the front desk, and your badge won't open the air shower, so you wait for someone to take you in.

The fabs, people and rules in the game are fictional; the cleanroom rules follow the procedures that universities and research labs publish and don't reflect any company's rules. For the jargon, see Chip fab jargon in plain English.