What does a mask engineer do in a chip fab?

In a chip fab, the mask engineer looks after every photomask (also called a reticle) in the building: receiving new ones from the plant that makes them, inspecting them, storing them, protecting them, taking them off for another check after a period of use, and arranging cleaning or repair when one gets dirty or damaged. A photomask is the "negative" used for exposure. How it prints its pattern onto a wafer is covered in What is photolithography?, so it isn't repeated here.

"Photomask engineer" can also mean a job at a plant that makes masks. This article is about the other side: the person who takes care of a mask once it has been delivered to the wafer fab.

Why does a photomask need someone to look after it?

Because a mask is a stamp. The scanner uses the same mask to stamp the wafer one field at a time until it is covered. One extra spot on the mask means one extra spot in the same place in every field.

And it doesn't stamp just one wafer. The same mask prints this lot today and the next lot tomorrow. A spot nobody catches gets printed lot after lot, many chips fail together, and yield drops.

Masks are also expensive. Each layer of a chip has its own mask, and a full set often runs to dozens. If one is ruined, that layer can't be made unless there is a spare. So a small mistake with a mask often costs far more than a mistake with one wafer.

What gets checked when a mask arrives?

Masks arrive from the mask shop in a dedicated box. On receipt, the mask engineer first checks three things:

• Is it the right one: the ID on the mask has to match.
• Is it clean: a mask inspection tool scans the whole plate. It marks particles, and anything on the pattern that shouldn't be there, as bright spots.
• Does it match the shipping report: the mask shop checked before shipping, but things can happen in transit.

Some fabs also expose one test wafer first, measure the line width and look for defects, and only let a full lot on once it looks right. That's called a print check.

What is the pellicle for?

A thin transparent film covers the mask. It is called a pellicle. A frame holds it up, a few millimeters above the pattern.

Those few millimeters matter. The scanner focuses on the pattern. Dust that lands on the film is too far from focus and prints as a blur, usually invisible; dust that lands right on the pattern prints sharp. The pellicle makes dust land "where it's out of focus". However clean a cleanroom is, its air still holds a few particles (see How clean is a cleanroom?), which is why the film is needed.

The pellicle needs care too:

• It is very thin and must not be touched directly.
• Exposure light slowly ages it, so it has a usage limit.
• If it gets scratched or torn, dust can reach the pattern, and it usually has to come off and be replaced.

Masks for extreme ultraviolet light are a different kind that reflects light, and their pellicles are even harder to make: the film has to let through light that is very easily absorbed, and it heats up under it. See What is extreme ultraviolet (EUV) light?.

Why does a mask get "haze"?

Haze is very fine crystals that slowly grow on a mask's surface after it has been in use for a while. It looks like fog on glass.

It isn't dust that fell on. It grows. The common explanation: tiny traces of chemicals left on the surface, such as ions left over from cleaning, meet the exposure light and moisture in the air, and slowly react into crystals. Studies found the main ingredient is ammonium sulfate. The more a mask is exposed, the faster it grows, and the shorter the wavelength of the exposure light, the bigger the problem.

At first the crystals only dim the light passing through, and the line width may drift a little. Once they grow, they print straight onto the wafer. So masks are kept in dedicated boxes and cabinets, as clean and dry as possible, to slow the growth.

Why check a mask again after it has been in use?

Because clean on arrival doesn't mean clean forever. Haze keeps growing; every time a mask is moved, loaded or unloaded, it may pick up particles; even static and electric fields can damage the fine lines on a mask.

So after a mask has been in use for a while, it is taken off and scanned again. Check too often and the mask spends more time out of use; check too rarely and a problem may only be found after it has grown. Researchers have also studied how often is worth it.

The wafer side helps too. If the check after developing finds the same spot in the same place in every field, that's called a repeating defect, and it almost always means the mask; a spot in only one field is more likely a particle on the wafer.

Each mask also has a history: when it arrived, how long the pellicle has been on, how much it has been exposed, what each inspection found.

What happens when a mask is dirty or damaged?

Stop using it, swap in the spare, then investigate. The mask comes off the scanner, a spare goes on if there is one, and the suspect mask goes under the inspection tool to be magnified. Then look back: which lot did the spot first appear in? Those lots go on hold first, and whoever owns them decides; wafers that haven't gone to etch yet can still be reworked.

What to do depends on what was found:

What you find	Usual fix
A small speck of dust on the pellicle	Usually doesn't print: note where it is and keep tracking it
A particle under the pellicle, on the pattern	Remove the pellicle, clean, mount a new pellicle, inspect again
Haze	Remove the pellicle, wash off the crystals, mount a new pellicle; often sent back to the mask shop for this
A scratched or torn pellicle	Mount a new pellicle
Extra or missing pattern	Won't wash off: the mask shop repairs it with a very fine ion beam or similar; if that fails, a new mask is made

Two things can't be skipped. First, a mask that has been cleaned, repaired or given a new pellicle must be scanned again, and sometimes print-checked, before it goes back. Second, a mask can't be cleaned forever: each wash may eat away a little of the film on the mask, and how much light gets through changes with it.

While a mask is away, that layer relies on its spare. With no spare, that layer's lots wait.

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 mask engineer (marked Beta). Most of your day is spent between the litho area and your desk; why that area is yellow is in Why is the lithography area in a chip fab yellow?. Which of the four shifts you get follows the time you open the game:

• Late-night shift, "Fog": you're this week's mask on-call. At 03:10, the mask inspection tool flags haze on a mask, and the next lot is waiting for it at the scanner. At home you look at the records and the images, and decide whether to keep it locked and use the spare, or let it go on first. At daybreak you write the wash form.
• Morning shift, "Back from the wash": a mask sent for cleaning two days ago is back, and the shipping report says it is clean. You do your own receiving inspection, line by line, and find one tiny particle under the pellicle. Do you tell the mask shop exactly what you found? A hot lot needs the mask at 10:30: print one wafer first or not?
• Afternoon shift, "A line on the film": there's a scratch on one mask's pellicle, and that layer has no spare. Under an oblique lamp you work out whether the scratch is on the film or on the pattern, then decide whether to keep using it, or send it for a new pellicle and stop that layer for two days.
• Evening shift, "The Same Spot": the litho process engineer calls: every exposure field has the same spot in the same place. Do you ask litho to swap in the spare first? Once the mask is off, it turns out to be a foreign particle: send it back to the mask shop, or wash it in the fab and verify it yourself? Last, you find the lot where it started.

The companies, the mask shop, the people, mask IDs, lot numbers and figures in the game are fictional; what a mask engineer looks after and how haze and pellicles work follow public, general sources, and how the work is split differs from company to company. For other jargon, see Chip fab jargon in plain English.